Add whether the vehicle is non-front to the vehicle pool pointer tag

Add iteration filters which check this

src/core/pool_type.hpp

@@ -117,12 +117,17 @@ struct Pool : PoolBase {
 	Pool(const char *name);
 	void CleanPool() override;
 
-	inline PtrType GetRaw(size_t index)
+	inline PtrType &GetRawRef(size_t index)
 	{
 		dbg_assert_msg(index < this->first_unused, "index: " PRINTF_SIZE ", first_unused: " PRINTF_SIZE ", name: %s", index, this->first_unused, this->name);
 		return this->data[index];
 	}
 
+	inline PtrType GetRaw(size_t index)
+	{
+		return this->GetRawRef(index);
+	}
+
 	/**
 	 * Returns Titem with given index
 	 * @param index of item to get

src/departures_gui.cpp

@@ -164,7 +164,7 @@ protected:
 		CompanyMask companies = 0;
 		int unitnumber_max[4] = { -1, -1, -1, -1 };
 
-		for (const Vehicle *v : Vehicle::Iterate()) {
+		for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 			if (v->type < 4 && this->show_types[v->type] && v->IsPrimaryVehicle()) {
 				for(const Order *order : v->Orders()) {
 					if ((order->IsType(OT_GOTO_STATION) || order->IsType(OT_GOTO_WAYPOINT) || order->IsType(OT_IMPLICIT))

src/disaster_vehicle.cpp

@@ -368,7 +368,7 @@ static bool DisasterTick_Ufo(DisasterVehicle *v)
 		}
 
 		n = RandomRange(n); // Choose one of them.
-		for (const RoadVehicle *u : RoadVehicle::Iterate()) {
+		for (const RoadVehicle *u : RoadVehicle::IterateFrontOnly()) {
 			/* Find (n+1)-th road vehicle. */
 			if (u->IsFrontEngine() && (n-- == 0)) {
 				if (u->crashed_ctr != 0 || !SetDisasterVehicleTargetingVehicle(u->index, v->index)) {

src/economy.cpp

@@ -236,7 +236,7 @@ int UpdateCompanyRatingAndValue(Company *c, bool update)
 		bool min_profit_first = true;
 		uint num = 0;
 
-		for (const Vehicle *v : Vehicle::Iterate()) {
+		for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 			if (v->owner != owner) continue;
 			if (IsCompanyBuildableVehicleType(v->type) && v->IsPrimaryVehicle() && !HasBit(v->subtype, GVSF_VIRTUAL)) {
 				if (v->profit_last_year > 0) num++; // For the vehicle score only count profitable vehicles

src/elrail.cpp

@@ -731,7 +731,7 @@ void SettingsDisableElrail(int32_t new_value)
 	}
 
 	/* Fix the total power and acceleration for trains */
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		/* power and acceleration is cached only for front engines */
 		if (t->IsFrontEngine()) {
 			t->ConsistChanged(CCF_TRACK);

src/group_cmd.cpp

@@ -220,7 +220,7 @@ uint16_t GroupStatistics::GetNumEngines(EngineID engine) const
 		g->statistics.ClearProfits();
 	}
 
-	for (const Vehicle *v : Vehicle::Iterate()) {
+	for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v->IsPrimaryVehicle() && !HasBit(v->subtype, GVSF_VIRTUAL)) {
 			GroupStatistics::AddProfitLastYear(v);
 			if (v->age > VEHICLE_PROFIT_MIN_AGE) GroupStatistics::VehicleReachedMinAge(v);
@@ -333,7 +333,7 @@ void IterateDescendantsOfGroup(GroupID id_top, F func)
 static void PropagateChildLiveryResetVehicleCache(const Group *g)
 {
 	/* Company colour data is indirectly cached. */
-	for (Vehicle *v : Vehicle::Iterate()) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v->IsPrimaryVehicle() && (v->group_id == g->index || IsGroupIDDescendantOfGroupID(v->group_id, g->index, g->owner))) {
 			for (Vehicle *u = v; u != nullptr; u = u->Next()) {
 				u->colourmap = PAL_NONE;
@@ -823,7 +823,7 @@ CommandCost CmdAddSharedVehicleGroup(TileIndex tile, DoCommandFlag flags, uint32
 	if (flags & DC_EXEC) {
 		/* Find the first front engine which belong to the group id_g
 		 * then add all shared vehicles of this front engine to the group id_g */
-		for (const Vehicle *v : Vehicle::IterateType(type)) {
+		for (const Vehicle *v : Vehicle::IterateTypeFrontOnly(type)) {
 			if (v->IsPrimaryVehicle()) {
 				if (v->group_id != id_g) continue;
 
@@ -860,7 +860,7 @@ CommandCost CmdRemoveAllVehiclesGroup(TileIndex tile, DoCommandFlag flags, uint3
 
 	if (flags & DC_EXEC) {
 		/* Find each Vehicle that belongs to the group old_g and add it to the default group */
-		for (const Vehicle *v : Vehicle::Iterate()) {
+		for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 			if (v->IsPrimaryVehicle()) {
 				if (v->group_id != old_g) continue;
 

src/infrastructure.cpp

@@ -206,7 +206,7 @@ static void FixAllReservations()
 {
 	/* if this function is called, we can safely assume that sharing of rails is being switched off */
 	assert(!_settings_game.economy.infrastructure_sharing[VEH_TRAIN]);
-	for (Train *v : Train::Iterate()) {
+	for (Train *v : Train::IterateFrontOnly()) {
 		if (!v->IsPrimaryVehicle() || (v->vehstatus & VS_CRASHED) != 0 || HasBit(v->subtype, GVSF_VIRTUAL)) continue;
 		/* It might happen that the train reserved additional tracks,
 		 * but FollowTrainReservation can't detect those because they are no longer reachable.
@@ -254,9 +254,8 @@ bool CheckSharingChangePossible(VehicleType type, bool new_value)
 	});
 
 	StringID error_message = STR_NULL;
-	for (Vehicle *v : Vehicle::IterateType(type)) {
+	for (Vehicle *v : Vehicle::IterateTypeFrontOnly(type)) {
 		if (HasBit(v->subtype, GVSF_VIRTUAL)) continue;
-		if (v->Previous() != nullptr) continue;
 
 		/* Check vehicle positiion */
 		if (!VehiclePositionIsAllowed(v)) {
@@ -280,7 +279,7 @@ bool CheckSharingChangePossible(VehicleType type, bool new_value)
 	}
 
 	if (type == VEH_TRAIN && _settings_game.vehicle.train_braking_model == TBM_REALISTIC) {
-		for (Train *v : Train::Iterate()) {
+		for (Train *v : Train::IterateFrontOnly()) {
 			if (!v->IsPrimaryVehicle() || (v->vehstatus & VS_CRASHED) != 0 || HasBit(v->subtype, GVSF_VIRTUAL)) continue;
 			/* It might happen that the train reserved additional tracks,
 			 * but FollowTrainReservation can't detect those because they are no longer reachable.
@@ -329,9 +328,9 @@ void HandleSharingCompanyDeletion(Owner owner)
 
 	Vehicle *si_v = nullptr;
 	SCOPE_INFO_FMT([&si_v], "HandleSharingCompanyDeletion: veh: %s", scope_dumper().VehicleInfo(si_v));
-	for (Vehicle *v : Vehicle::Iterate()) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 		si_v = v;
-		if (!IsCompanyBuildableVehicleType(v) || v->Previous() != nullptr) continue;
+		if (!IsCompanyBuildableVehicleType(v)) continue;
 		/* vehicle position */
 		if (v->owner == owner || !VehiclePositionIsAllowed(v, owner)) {
 			RemoveAndSellVehicle(v, v->owner != owner);

src/network/network_server.cpp

@@ -1759,7 +1759,7 @@ void NetworkPopulateCompanyStats(NetworkCompanyStats *stats)
 	memset(stats, 0, sizeof(*stats) * MAX_COMPANIES);
 
 	/* Go through all vehicles and count the type of vehicles */
-	for (const Vehicle *v : Vehicle::Iterate()) {
+	for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (!Company::IsValidID(v->owner) || !v->IsPrimaryVehicle() || HasBit(v->subtype, GVSF_VIRTUAL)) continue;
 		byte type = 0;
 		switch (v->type) {

src/order_cmd.cpp

@@ -66,9 +66,9 @@ CommandCost CmdInsertOrderIntl(DoCommandFlag flags, Vehicle *v, VehicleOrderID s
 void IntialiseOrderDestinationRefcountMap()
 {
 	ClearOrderDestinationRefcountMap();
-	for (const Vehicle *v : Vehicle::Iterate()) {
+	for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v != v->FirstShared()) continue;
-		for(const Order *order : v->Orders()) {
+		for (const Order *order : v->Orders()) {
 			if (order->IsType(OT_GOTO_STATION) || order->IsType(OT_GOTO_WAYPOINT) || order->IsType(OT_IMPLICIT)) {
 				_order_destination_refcount_map[OrderDestinationRefcountMapKey(order->GetDestination(), v->owner, order->GetType(), v->type)]++;
 			}
@@ -2933,7 +2933,7 @@ void RemoveOrderFromAllVehicles(OrderType type, DestinationID destination, bool
 	 */
 
 	/* Go through all vehicles */
-	for (Vehicle *v : Vehicle::Iterate()) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 		Order *order = &v->current_order;
 		if ((v->type == VEH_AIRCRAFT && order->IsType(OT_GOTO_DEPOT) && !hangar ? OT_GOTO_STATION : order->GetType()) == type &&
 				(!hangar || v->type == VEH_AIRCRAFT) && order->GetDestination() == destination) {
@@ -3786,7 +3786,7 @@ CommandCost CmdMassChangeOrder(TileIndex tile, DoCommandFlag flags, uint32_t p1,
 	DestinationID to_dest = GB(p2, 0, 16);
 
 	if (flags & DC_EXEC) {
-		for (Vehicle *v : Vehicle::IterateType(vehtype)) {
+		for (Vehicle *v : Vehicle::IterateTypeFrontOnly(vehtype)) {
 			if (v->IsPrimaryVehicle() && CheckOwnership(v->owner).Succeeded() && VehicleCargoFilter(v, cargo_filter)) {
 				int index = 0;
 				bool changed = false;

src/road_cmd.cpp

@@ -203,7 +203,7 @@ bool RoadVehiclesAreBuilt()
  */
 bool RoadVehiclesExistOutsideDepots()
 {
-	for (const RoadVehicle *rv : RoadVehicle::Iterate()) {
+	for (const RoadVehicle *rv : RoadVehicle::IterateFrontOnly()) {
 		if (rv->IsFrontEngine() && !rv->IsChainInDepot()) return true;
 	}
 	return false;

src/saveload/afterload.cpp

@@ -308,10 +308,8 @@ static void InitializeWindowsAndCaches()
 			it->tile = t->xy;
 		}
 	}
-	for (RoadVehicle *rv : RoadVehicle::Iterate()) {
-		if (rv->IsFrontEngine()) {
-			rv->CargoChanged();
-		}
+	for (RoadVehicle *rv : RoadVehicle::IterateFrontOnly()) {
+		rv->CargoChanged();
 	}
 
 	RecomputePrices();
@@ -1735,7 +1733,7 @@ bool AfterLoadGame()
 			}
 		}
 
-		for (Train *v : Train::Iterate()) {
+		for (Train *v : Train::IterateFrontOnly()) {
 			if (v->IsFrontEngine() || v->IsFreeWagon()) v->ConsistChanged(CCF_TRACK);
 		}
 
@@ -2370,8 +2368,8 @@ bool AfterLoadGame()
 		GroupStatistics::UpdateAfterLoad(); // Ensure statistics pool is initialised before trying to delete vehicles
 		/* Remove all trams from savegames without tram support.
 		 * There would be trams without tram track under causing crashes sooner or later. */
-		for (RoadVehicle *v : RoadVehicle::Iterate()) {
-			if (v->First() == v && HasBit(EngInfo(v->engine_type)->misc_flags, EF_ROAD_TRAM)) {
+		for (RoadVehicle *v : RoadVehicle::IterateFrontOnly()) {
+			if (HasBit(EngInfo(v->engine_type)->misc_flags, EF_ROAD_TRAM)) {
 				ShowErrorMessage(STR_WARNING_LOADGAME_REMOVED_TRAMS, INVALID_STRING_ID, WL_CRITICAL);
 				delete v;
 			}
@@ -2466,8 +2464,8 @@ bool AfterLoadGame()
 
 	/* Reserve all tracks trains are currently on. */
 	if (IsSavegameVersionBefore(SLV_101)) {
-		for (const Train *t : Train::Iterate()) {
-			if (t->First() == t) t->ReserveTrackUnderConsist();
+		for (const Train *t : Train::IterateFrontOnly()) {
+			t->ReserveTrackUnderConsist();
 		}
 	}
 
@@ -3196,7 +3194,7 @@ bool AfterLoadGame()
 		}
 
 		/* Fill Vehicle::cur_real_order_index */
-		for (Vehicle *v : Vehicle::Iterate()) {
+		for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 			if (!v->IsPrimaryVehicle()) continue;
 
 			/* Older versions are less strict with indices being in range and fix them on the fly */
@@ -3501,7 +3499,7 @@ bool AfterLoadGame()
 		 * So, make articulated parts catch up. */
 		bool roadside = _settings_game.vehicle.road_side == 1;
 		std::vector<uint> skip_frames;
-		for (RoadVehicle *v : RoadVehicle::Iterate()) {
+		for (RoadVehicle *v : RoadVehicle::IterateFrontOnly()) {
 			if (!v->IsFrontEngine()) continue;
 			skip_frames.clear();
 			TileIndex prev_tile = v->tile;
@@ -3706,7 +3704,7 @@ bool AfterLoadGame()
 	}
 
 	if (!IsSavegameVersionBefore(SLV_DEPOT_UNBUNCHING)) {
-		for (Vehicle *v : Vehicle::Iterate()) {
+		for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 			if (v->unbunch_state != nullptr) {
 				if (v->unbunch_state->depot_unbunching_last_departure > 0) {
 					v->unbunch_state->depot_unbunching_last_departure += _state_ticks.base() - _tick_counter;
@@ -4217,7 +4215,7 @@ bool AfterLoadGame()
 	}
 
 	if (!SlXvIsFeaturePresent(XSLFI_REALISTIC_TRAIN_BRAKING, 5) && _settings_game.vehicle.train_braking_model == TBM_REALISTIC) {
-		for (Train *t : Train::Iterate()) {
+		for (Train *t : Train::IterateFrontOnly()) {
 			if (t->lookahead != nullptr) {
 				t->lookahead->SetNextExtendPosition();
 			}
@@ -4225,7 +4223,7 @@ bool AfterLoadGame()
 	}
 
 	if (!SlXvIsFeaturePresent(XSLFI_REALISTIC_TRAIN_BRAKING, 6) && _settings_game.vehicle.train_braking_model == TBM_REALISTIC) {
-		for (Train *t : Train::Iterate()) {
+		for (Train *t : Train::IterateFrontOnly()) {
 			if (t->lookahead != nullptr) {
 				t->lookahead->cached_zpos = t->CalculateOverallZPos();
 				t->lookahead->zpos_refresh_remaining = t->GetZPosCacheUpdateInterval();
@@ -4366,7 +4364,7 @@ bool AfterLoadGame()
 	}
 
 	if (!SlXvIsFeaturePresent(XSLFI_REALISTIC_TRAIN_BRAKING, 9) && _settings_game.vehicle.train_braking_model == TBM_REALISTIC) {
-		for (Train *t : Train::Iterate()) {
+		for (Train *t : Train::IterateFrontOnly()) {
 			if (t->lookahead != nullptr) {
 				t->lookahead->lookahead_end_position = t->lookahead->reservation_end_position + 1;
 			}

src/script/api/script_group.cpp

@@ -180,7 +180,7 @@
 
 	Money profit = 0;
 
-	for (const Vehicle *v : Vehicle::Iterate()) {
+	for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v->group_id != group_id) continue;
 		if (!v->IsPrimaryVehicle()) continue;
 
@@ -204,7 +204,7 @@
 	uint32_t occupancy = 0;
 	uint32_t vehicle_count = 0;
 
-	for (const Vehicle *v : Vehicle::Iterate()) {
+	for (const Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v->group_id != group_id) continue;
 		if (!v->IsPrimaryVehicle()) continue;
 

src/settings.cpp

@@ -1141,7 +1141,7 @@ static void StationSpreadChanged(int32_t new_value)
 
 static void UpdateConsists(int32_t new_value)
 {
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		/* Update the consist of all trains so the maximum speed is set correctly. */
 		if (t->IsFrontEngine() || t->IsFreeWagon()) {
 			t->ConsistChanged(CCF_TRACK);
@@ -1196,7 +1196,7 @@ static void UpdateAllServiceInterval(int32_t new_value)
 
 	if (update_vehicles) {
 		const Company *c = Company::Get(_current_company);
-		for (Vehicle *v : Vehicle::Iterate()) {
+		for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 			if (v->owner == _current_company && v->IsPrimaryVehicle() && !v->ServiceIntervalIsCustom()) {
 				v->SetServiceInterval(CompanyServiceInterval(c, v->type));
 				v->SetServiceIntervalIsPercent(new_value != 0);
@@ -1224,7 +1224,7 @@ static bool CanUpdateServiceInterval(VehicleType type, int32_t &new_value)
 static void UpdateServiceInterval(VehicleType type, int32_t new_value)
 {
 	if (_game_mode != GM_MENU && Company::IsValidID(_current_company)) {
-		for (Vehicle *v : Vehicle::IterateType(type)) {
+		for (Vehicle *v : Vehicle::IterateTypeFrontOnly(type)) {
 			if (v->owner == _current_company && v->IsPrimaryVehicle() && !v->ServiceIntervalIsCustom()) {
 				v->SetServiceInterval(new_value);
 			}
@@ -1330,7 +1330,7 @@ static void ChangeMinutesPerYear(int32_t new_value)
 
 static void TrainAccelerationModelChanged(int32_t new_value)
 {
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		if (t->IsFrontEngine()) {
 			t->tcache.cached_max_curve_speed = t->GetCurveSpeedLimit();
 			t->UpdateAcceleration();
@@ -1406,13 +1406,13 @@ static void TrainBrakingModelChanged(int32_t new_value)
 		SCOPE_INFO_FMT([&v_cur], "TrainBrakingModelChanged: %s", scope_dumper().VehicleInfo(v_cur));
 		extern bool _long_reserve_disabled;
 		_long_reserve_disabled = true;
-		for (Train *v : Train::Iterate()) {
+		for (Train *v : Train::IterateFrontOnly()) {
 			v_cur = v;
 			if (!v->IsPrimaryVehicle() || (v->vehstatus & VS_CRASHED) != 0 || HasBit(v->subtype, GVSF_VIRTUAL) || v->track == TRACK_BIT_DEPOT) continue;
 			TryPathReserve(v, true, HasStationTileRail(v->tile));
 		}
 		_long_reserve_disabled = false;
-		for (Train *v : Train::Iterate()) {
+		for (Train *v : Train::IterateFrontOnly()) {
 			v_cur = v;
 			if (!v->IsPrimaryVehicle() || (v->vehstatus & VS_CRASHED) != 0 || HasBit(v->subtype, GVSF_VIRTUAL) || v->track == TRACK_BIT_DEPOT) continue;
 			TryPathReserve(v, true, HasStationTileRail(v->tile));
@@ -1421,7 +1421,7 @@ static void TrainBrakingModelChanged(int32_t new_value)
 	} else if (new_value == TBM_ORIGINAL && (_game_mode == GM_NORMAL || _game_mode == GM_EDITOR)) {
 		Train *v_cur = nullptr;
 		SCOPE_INFO_FMT([&v_cur], "TrainBrakingModelChanged: %s", scope_dumper().VehicleInfo(v_cur));
-		for (Train *v : Train::Iterate()) {
+		for (Train *v : Train::IterateFrontOnly()) {
 			v_cur = v;
 			if (!v->IsPrimaryVehicle() || (v->vehstatus & VS_CRASHED) != 0 || HasBit(v->subtype, GVSF_VIRTUAL) || v->track == TRACK_BIT_DEPOT) {
 				v->lookahead.reset();
@@ -1451,7 +1451,7 @@ static void TrainBrakingModelChanged(int32_t new_value)
  */
 static void TrainSlopeSteepnessChanged(int32_t new_value)
 {
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		if (t->IsFrontEngine()) {
 			t->CargoChanged();
 			if (t->lookahead != nullptr) SetBit(t->lookahead->flags, TRLF_APPLY_ADVISORY);
@@ -1466,17 +1466,13 @@ static void TrainSlopeSteepnessChanged(int32_t new_value)
 static void RoadVehAccelerationModelChanged(int32_t new_value)
 {
 	if (_settings_game.vehicle.roadveh_acceleration_model != AM_ORIGINAL) {
-		for (RoadVehicle *rv : RoadVehicle::Iterate()) {
-			if (rv->IsFrontEngine()) {
-				rv->CargoChanged();
-			}
+		for (RoadVehicle *rv : RoadVehicle::IterateFrontOnly()) {
+			rv->CargoChanged();
 		}
 	}
 	if (_settings_game.vehicle.roadveh_acceleration_model == AM_ORIGINAL || !_settings_game.vehicle.improved_breakdowns) {
-		for (RoadVehicle *rv : RoadVehicle::Iterate()) {
-			if (rv->IsFrontEngine()) {
-				rv->breakdown_chance_factor = 128;
-			}
+		for (RoadVehicle *rv : RoadVehicle::IterateFrontOnly()) {
+			rv->breakdown_chance_factor = 128;
 		}
 	}
 
@@ -1493,8 +1489,8 @@ static void RoadVehAccelerationModelChanged(int32_t new_value)
  */
 static void RoadVehSlopeSteepnessChanged(int32_t new_value)
 {
-	for (RoadVehicle *rv : RoadVehicle::Iterate()) {
-		if (rv->IsFrontEngine()) rv->CargoChanged();
+	for (RoadVehicle *rv : RoadVehicle::IterateFrontOnly()) {
+		rv->CargoChanged();
 	}
 }
 
@@ -2022,8 +2018,8 @@ static void ImprovedBreakdownsSettingChanged(int32_t new_value)
 {
 	if (!_settings_game.vehicle.improved_breakdowns) return;
 
-	for (Vehicle *v : Vehicle::Iterate()) {
-		switch(v->type) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
+		switch (v->type) {
 			case VEH_TRAIN:
 				if (v->IsFrontEngine()) {
 					v->breakdown_chance_factor = 128;

src/sl/vehicle_sl.cpp

@@ -273,6 +273,9 @@ void AfterLoadVehicles(bool part_of_load)
 		/* Reinstate the previous pointer */
 		if (v->Next() != nullptr) {
 			v->Next()->previous = v;
+#if OTTD_UPPER_TAGGED_PTR
+			VehiclePoolOps::SetIsNonFrontVehiclePtr(_vehicle_pool.GetRawRef(v->Next()->index), true);
+#endif
 			if (v->type == VEH_TRAIN && (HasBit(v->subtype, GVSF_VIRTUAL) != HasBit(v->Next()->subtype, GVSF_VIRTUAL))) {
 				SlErrorCorrupt("Mixed virtual/non-virtual train consist");
 			}
@@ -387,7 +390,7 @@ void AfterLoadVehicles(bool part_of_load)
 
 		if (IsSavegameVersionBefore(SLV_180)) {
 			/* Set service interval flags */
-			for (Vehicle *v : Vehicle::Iterate()) {
+			for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 				si_v = v;
 				if (!v->IsPrimaryVehicle()) continue;
 
@@ -429,7 +432,7 @@ void AfterLoadVehicles(bool part_of_load)
 
 	CheckValidVehicles();
 
-	for (Vehicle *v : Vehicle::Iterate()) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 		si_v = v;
 		assert(v->first != nullptr);
 
@@ -595,7 +598,7 @@ void FixupTrainLengths()
 {
 	/* Vehicle center was moved from 4 units behind the front to half the length
 	 * behind the front. Move vehicles so they end up on the same spot. */
-	for (Train *v : Train::Iterate()) {
+	for (Train *v : Train::IterateFrontOnly()) {
 		if (v->IsPrimaryVehicle()) {
 			/* The vehicle center is now more to the front depending on vehicle length,
 			 * so we need to move all vehicles forward to cover the difference to the
@@ -1622,7 +1625,7 @@ const SaveLoadTable GetVehicleUnbunchStateDescription()
 
 void Save_VUBS()
 {
-	for (Vehicle *v : Vehicle::Iterate()) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v->unbunch_state != nullptr) {
 			SlSetArrayIndex(v->index);
 			SlObject(v->unbunch_state.get(), GetVehicleUnbunchStateDescription());

src/station_cmd.cpp

@@ -2456,9 +2456,8 @@ CommandCost RemoveRoadStop(TileIndex tile, DoCommandFlag flags, int replacement_
 		delete cur_stop;
 
 		/* Make sure no vehicle is going to the old roadstop */
-		for (RoadVehicle *v : RoadVehicle::Iterate()) {
-			if (v->First() == v && v->current_order.IsType(OT_GOTO_STATION) &&
-					v->dest_tile == tile) {
+		for (RoadVehicle *v : RoadVehicle::IterateFrontOnly()) {
+			if (v->current_order.IsType(OT_GOTO_STATION) && v->dest_tile == tile) {
 				v->SetDestTile(v->GetOrderStationLocation(st->index));
 			}
 		}
@@ -3236,9 +3235,7 @@ static CommandCost RemoveDock(TileIndex tile, DoCommandFlag flags)
 		ClearDockingTilesCheckingNeighbours(tile1);
 		ClearDockingTilesCheckingNeighbours(tile2);
 
-		for (Ship *s : Ship::Iterate()) {
-			if (!s->IsPrimaryVehicle()) continue;
-
+		for (Ship *s : Ship::IterateFrontOnly()) {
 			/* Find all ships going to our dock. */
 			if (s->current_order.GetDestination() != st->index) {
 				continue;

src/tbtr_template_vehicle.cpp

@@ -204,7 +204,7 @@ static void MarkTrainsInGroupAsPendingTemplateReplacement(GroupID gid, const Tem
 
 	std::sort(groups.begin(), groups.end());
 
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		if (!t->IsFrontEngine() || t->owner != owner || t->group_id >= NEW_GROUP) continue;
 
 		if (std::binary_search(groups.begin(), groups.end(), t->group_id)) {
@@ -217,7 +217,7 @@ void MarkTrainsUsingTemplateAsPendingTemplateReplacement(const TemplateVehicle *
 {
 	Owner owner = tv->owner;
 
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		if (!t->IsFrontEngine() || t->owner != owner || t->group_id >= NEW_GROUP) continue;
 
 		if (GetTemplateIDByGroupIDRecursive(t->group_id) == tv->index) {

src/tbtr_template_vehicle_func.cpp

@@ -327,7 +327,7 @@ uint CountsTrainsNeedingTemplateReplacement(GroupID g_id, const TemplateVehicle
 	uint count = 0;
 	if (!tv) return count;
 
-	for (Train *t : Train::Iterate()) {
+	for (Train *t : Train::IterateFrontOnly()) {
 		if (t->IsPrimaryVehicle() && t->group_id == g_id && TrainTemplateDifference(t, tv) != TBTRDF_NONE) {
 			count++;
 		}

src/train_cmd.cpp

@@ -170,8 +170,8 @@ void CheckTrainsLengths()
 {
 	bool first = true;
 
-	for (const Train *v : Train::Iterate()) {
-		if (v->First() == v && !(v->vehstatus & VS_CRASHED) && !v->IsVirtual()) {
+	for (const Train *v : Train::IterateFrontOnly()) {
+		if (!(v->vehstatus & VS_CRASHED) && !v->IsVirtual()) {
 			for (const Train *u = v, *w = v->Next(); w != nullptr; u = w, w = w->Next()) {
 				if (u->track != TRACK_BIT_DEPOT) {
 					if ((w->track != TRACK_BIT_DEPOT &&
@@ -1503,7 +1503,7 @@ static CommandCost CmdBuildRailWagon(TileIndex tile, DoCommandFlag flags, const
 		CheckConsistencyOfArticulatedVehicle(v);
 
 		/* Try to connect the vehicle to one of free chains of wagons. */
-		for (Train *w : Train::Iterate()) {
+		for (Train *w : Train::IterateFrontOnly()) {
 			if (w->tile == tile &&              ///< Same depot
 					w->IsFreeWagon() &&             ///< A free wagon chain
 					w->engine_type == e->index &&   ///< Same type
@@ -1527,7 +1527,7 @@ static CommandCost CmdBuildRailWagon(TileIndex tile, DoCommandFlag flags, const
 void NormalizeTrainVehInDepot(const Train *u)
 {
 	assert(u->IsEngine());
-	for (const Train *v : Train::Iterate()) {
+	for (const Train *v : Train::IterateFrontOnly()) {
 		if (v->IsFreeWagon() && v->tile == u->tile &&
 				v->track == TRACK_BIT_DEPOT &&
 				v->owner == u->owner &&
@@ -1674,7 +1674,7 @@ static Train *FindGoodVehiclePos(const Train *src)
 	EngineID eng = src->engine_type;
 	TileIndex tile = src->tile;
 
-	for (Train *dst : Train::Iterate()) {
+	for (Train *dst : Train::IterateFrontOnly()) {
 		if (dst->IsFreeWagon() && dst->tile == tile && !(dst->vehstatus & VS_CRASHED) && dst->owner == src->owner && !dst->IsVirtual()) {
 			/* check so all vehicles in the line have the same engine. */
 			Train *t = dst;

src/train_gui.cpp

@@ -36,7 +36,7 @@ void CcBuildWagon(const CommandCost &result, TileIndex tile, uint32_t p1, uint32
 
 	/* find a locomotive in the depot. */
 	const Vehicle *found = nullptr;
-	for (const Train *t : Train::Iterate()) {
+	for (const Train *t : Train::IterateFrontOnly()) {
 		if (t->IsFrontEngine() && t->tile == tile && t->IsStoppedInDepot() && !t->IsVirtual()) {
 			if (found != nullptr) return; // must be exactly one.
 			found = t;

src/tunnelbridge_cmd.cpp

@@ -2861,7 +2861,7 @@ static bool ClickTile_TunnelBridge(TileIndex tile)
 	if (IsTunnelTile(tile)) {
 		int count = 0;
 		TileIndex tile_end = GetOtherTunnelBridgeEnd(tile);
-		for (const Train *t : Train::Iterate()) {
+		for (const Train *t : Train::IterateFrontOnly()) {
 			if (!t->IsFrontEngine()) continue;
 			if (tile == t->tile || tile_end == t->tile) {
 				ShowVehicleViewWindow(t);

src/vehicle.cpp

@@ -4317,6 +4317,9 @@ void Vehicle::SetNext(Vehicle *next)
 			v->first = this->next;
 		}
 		this->next->previous = nullptr;
+#if OTTD_UPPER_TAGGED_PTR
+		VehiclePoolOps::SetIsNonFrontVehiclePtr(_vehicle_pool.GetRawRef(this->next->index), false);
+#endif
 	}
 
 	this->next = next;
@@ -4325,6 +4328,9 @@ void Vehicle::SetNext(Vehicle *next)
 		/* A new next vehicle. Update the first and previous pointers */
 		if (this->next->previous != nullptr) this->next->previous->next = nullptr;
 		this->next->previous = this;
+#if OTTD_UPPER_TAGGED_PTR
+		VehiclePoolOps::SetIsNonFrontVehiclePtr(_vehicle_pool.GetRawRef(this->next->index), true);
+#endif
 		for (Vehicle *v = this->next; v != nullptr; v = v->Next()) {
 			v->first = this->first;
 		}
@@ -4549,7 +4555,7 @@ char *Vehicle::DumpVehicleFlagsMultiline(char *b, const char *last, const char *
 
 void VehiclesYearlyLoop()
 {
-	for (Vehicle *v : Vehicle::Iterate()) {
+	for (Vehicle *v : Vehicle::IterateFrontOnly()) {
 		if (v->IsPrimaryVehicle()) {
 			/* show warning if vehicle is not generating enough income last 2 years (corresponds to a red icon in the vehicle list) */
 			Money profit = v->GetDisplayProfitThisYear();

src/vehicle_base.h

@@ -255,6 +255,9 @@ struct VehiclePoolOps {
 	static constexpr VehicleType DefaultItemParam() { return VEH_INVALID; }
 
 	static constexpr VehicleType GetVehicleType(uintptr_t ptr) { return static_cast<VehicleType>(GB(ptr, 60, 3)); }
+	static constexpr bool IsNonFrontVehiclePtr(uintptr_t ptr) { return HasBit(ptr, 63); }
+
+	static constexpr void SetIsNonFrontVehiclePtr(uintptr_t &ptr, bool non_front) { SB(ptr, 63, 1, non_front ? 1 : 0); }
 };
 
 typedef Pool<Vehicle, VehicleID, 512, 0xFF000, PT_NORMAL, false, true, VehiclePoolOps> VehiclePool;
@@ -1303,6 +1306,32 @@ public:
 		}
 	};
 
+	struct VehicleFrontOnlyFilter {
+		bool operator() (size_t index)
+		{
+#if OTTD_UPPER_TAGGED_PTR
+			return !VehiclePoolOps::IsNonFrontVehiclePtr(_vehicle_pool.GetRaw(index));
+#else
+			return Vehicle::Get(index)->Previous() == nullptr;
+#endif
+		}
+	};
+
+	struct VehicleFrontOnlyTypeFilter {
+		VehicleType vt;
+
+		bool operator() (size_t index)
+		{
+#if OTTD_UPPER_TAGGED_PTR
+			uintptr_t vptr = _vehicle_pool.GetRaw(index);
+			return !VehiclePoolOps::IsNonFrontVehiclePtr(vptr) && VehiclePoolOps::GetVehicleType(vptr) == this->vt;
+#else
+			const Vehicle *v = Vehicle::Get(index);
+			return v->type == this->vt && v->Previous() == nullptr;
+#endif
+		}
+	};
+
 	/**
 	 * Returns an iterable ensemble of all valid vehicles of the given type
 	 * @param vt the VehicleType to filter
@@ -1313,6 +1342,27 @@ public:
 	{
 		return Pool::IterateWrapperFiltered<Vehicle, VehicleTypeFilter>(from, VehicleTypeFilter{ vt });
 	}
+
+	/**
+	 * Returns an iterable ensemble of all valid front vehicles (i.e. Previous() == nullptr)
+	 * @param from index of the first vehicle to consider
+	 * @return an iterable ensemble of all valid front vehicles
+	 */
+	static Pool::IterateWrapperFiltered<Vehicle, VehicleFrontOnlyFilter> IterateFrontOnly(size_t from = 0)
+	{
+		return Pool::IterateWrapperFiltered<Vehicle, VehicleFrontOnlyFilter>(from, VehicleFrontOnlyFilter{});
+	}
+
+	/**
+	 * Returns an iterable ensemble of all valid front vehicles of the given type
+	 * @param vt the VehicleType to filter
+	 * @param from index of the first vehicle to consider
+	 * @return an iterable ensemble of all valid front vehicles of the given type
+	 */
+	static Pool::IterateWrapperFiltered<Vehicle, VehicleFrontOnlyTypeFilter> IterateTypeFrontOnly(VehicleType vt, size_t from = 0)
+	{
+		return Pool::IterateWrapperFiltered<Vehicle, VehicleFrontOnlyTypeFilter>(from, VehicleFrontOnlyTypeFilter{ vt });
+	}
 };
 
 inline bool IsPointInViewportVehicleRedrawArea(const std::vector<Rect> &viewport_redraw_rects, const Point &pt)
@@ -1608,6 +1658,16 @@ public:
 	 * @return an iterable ensemble of all valid vehicles of type T
 	 */
 	static Pool::IterateWrapper<T> Iterate(size_t from = 0) { return Pool::IterateWrapper<T>(from); }
+
+	/**
+	 * Returns an iterable ensemble of all valid front vehicles (i.e. Previous() == nullptr) of type T
+	 * @param from index of the first vehicle to consider
+	 * @return an iterable ensemble of all valid front vehicles of type T
+	 */
+	static Pool::IterateWrapperFiltered<T, VehicleFrontOnlyFilter> IterateFrontOnly(size_t from = 0)
+	{
+		return Pool::IterateWrapperFiltered<T, VehicleFrontOnlyFilter>(from, VehicleFrontOnlyFilter{});
+	}
 };
 
 /** Generates sequence of free UnitID numbers */

src/vehiclelist.cpp

@@ -155,7 +155,7 @@ bool GenerateVehicleSortList(VehicleList *list, const VehicleListIdentifier &vli
 	};
 
 	auto fill_all_vehicles = [&]() {
-		for (const Vehicle *v : Vehicle::IterateType(vli.vtype)) {
+		for (const Vehicle *v : Vehicle::IterateTypeFrontOnly(vli.vtype)) {
 			if (!HasBit(v->subtype, GVSF_VIRTUAL) && v->owner == vli.company && v->IsPrimaryVehicle()) {
 				add_veh(v);
 			}
@@ -184,7 +184,7 @@ bool GenerateVehicleSortList(VehicleList *list, const VehicleListIdentifier &vli
 
 		case VL_GROUP_LIST:
 			if (vli.index != ALL_GROUP) {
-				for (const Vehicle *v : Vehicle::IterateType(vli.vtype)) {
+				for (const Vehicle *v : Vehicle::IterateTypeFrontOnly(vli.vtype)) {
 					if (!HasBit(v->subtype, GVSF_VIRTUAL) && v->IsPrimaryVehicle() &&
 							v->owner == vli.company && GroupIsInGroup(v->group_id, vli.index)) {
 						add_veh(v);