Fix speed calculation

src/train_gui.cpp

@@ -405,6 +405,65 @@ int GetTrainDetailsWndVScroll(VehicleID veh_id, TrainDetailsWindowTabs det_tab)
 	return num;
 }
 
+int GetAcceleration(const Train *train, const int speed, const int mass)
+{
+	const int64 power = train->gcache.cached_power * 746ll;
+	int64 resistance = 0;
+
+	const bool maglev = (GetRailTypeInfo(train->railtype)->acceleration_type == 2);
+
+	if (!maglev) {
+		/* Static resistance plus rolling friction. */
+		resistance = 10 * mass;
+		resistance += mass * (15 * (512 + speed) / 512);
+	}
+
+	const int area = 14;
+
+	resistance += (area * train->gcache.cached_air_drag * speed * speed) / 1000;
+
+	uint32 max_te = train->gcache.cached_max_te; // [N]
+	int64 force;
+
+	if (speed > 0) {
+		if (!maglev) {
+			/* Conversion factor from km/h to m/s is 5/18 to get [N] in the end. */
+			force = power * 18 / (speed * 5);
+
+			if (force > static_cast<int>(max_te)) {
+				force = max_te;
+			}
+		} else {
+			force = power / 25;
+		}
+	} else {
+		force = (!maglev) ? std::min<uint64>(max_te, power) : power;
+		force = std::max(force, (mass * 8) + resistance);
+	}
+
+	/* Easy way out when there is no acceleration. */
+	if (force == resistance) return 0;
+
+	int acceleration = ClampToI32((force - resistance) / (mass * 4));
+	acceleration = force < resistance ? std::min(-1, acceleration) : std::max(1, acceleration);
+
+	return acceleration;
+}
+
+int GetMaxSpeed(const Train *train, const int mass, const int speed_cap)
+{
+	int max_speed = 0;
+	int acceleration;
+	
+	do
+	{
+		max_speed++;
+		acceleration = GetAcceleration(train, max_speed, mass);
+	} while (acceleration > 0 && max_speed < speed_cap);
+
+	return max_speed;
+}
+
 /**
  * Draw the details for the given vehicle at the given position
  *
@@ -514,8 +573,6 @@ void DrawTrainDetails(const Train *v, int left, int right, int y, int vscroll_po
 		Money feeder_share = 0;
 		int empty_weight = 0;
 		int loaded_weight = 0;
-		int empty_max_speed = 0;
-		int loaded_max_speed = 0;
 
 		for (const Vehicle *u = v; u != nullptr; u = u->Next()) {
 			const Train *train = Train::From(u);
@@ -527,13 +584,8 @@ void DrawTrainDetails(const Train *v, int left, int right, int y, int vscroll_po
 			loaded_weight            += weight_without_cargo + train->GetCargoWeight(train->cargo_cap);
 		}
 
-		const auto rolling_friction = 15 * (512 + v->GetDisplayMaxSpeed()) / 512;
-		const auto tractive_effort_empty = empty_weight * rolling_friction;
-		const auto tractive_effort_loaded = loaded_weight * rolling_friction;
-		const int power = static_cast<int>(v->gcache.cached_power * 746);
-		const int max_te = static_cast<int>(v->gcache.cached_max_te);
-		empty_max_speed = std::min(v->GetDisplayMaxSpeed(), (tractive_effort_empty == 0 || tractive_effort_empty > max_te) ? 0 : static_cast<int>((3.6 * power) / tractive_effort_empty));
-		loaded_max_speed = std::min(v->GetDisplayMaxSpeed(), (tractive_effort_loaded == 0 || tractive_effort_loaded > max_te) ? 0 : static_cast<int>((3.6 * power) / tractive_effort_loaded));
+		const int empty_max_speed = GetMaxSpeed(v, empty_weight, v->GetDisplayMaxSpeed());
+		const int loaded_max_speed = GetMaxSpeed(v, loaded_weight, v->GetDisplayMaxSpeed());
 
 		if (--vscroll_pos < 0 && vscroll_pos >= -vscroll_cap) {
 			SetDParam(0, empty_weight);