Add setting text/warnings about why YAPF is recommended for trains

src/lang/extra/english.txt

@@ -775,6 +775,8 @@ STR_CONFIG_SETTING_NETWORK_CHANGE_NOT_ALLOWED_NEWGRF            :{WHITE}...setti
 STR_CONFIG_SETTING_TIMETABLE_IN_TICKS                           :Show timetable in ticks rather than days: {STRING2}
 STR_CONFIG_SETTING_TIMETABLE_IN_TICKS_HELPTEXT                  :Show travel times in time tables in game ticks instead of days
 
+STR_CONFIG_SETTING_PATHFINDER_FOR_TRAINS_HELPTEXT_EXTRA         :{STRING}{}Various features require use of the YAPF pathfinder, including: routing restrictions (all pathfinder and reservation related actions), reverse at waypoint/signal and drive through train depots.
+
 STR_CHEAT_EDIT_MONEY_QUERY_CAPT                                 :{WHITE}Increase or decrease money
 STR_CHEAT_INFLATION_COST                                        :{LTBLUE}Change inflation cost factor: {ORANGE}{DECIMAL}
 STR_CHEAT_INFLATION_COST_QUERY_CAPT                             :{WHITE}Change inflation cost factor

src/settings.cpp

@@ -2026,6 +2026,33 @@ static bool ChunnelSettingGUI(SettingOnGuiCtrlData &data)
 	}
 }
 
+static bool TrainPathfinderSettingGUI(SettingOnGuiCtrlData &data)
+{
+	switch (data.type) {
+		case SOGCT_DESCRIPTION_TEXT:
+			SetDParam(0, data.text);
+			data.text = STR_CONFIG_SETTING_PATHFINDER_FOR_TRAINS_HELPTEXT_EXTRA;
+			return true;
+
+		case SOGCT_GUI_SPRITE:
+			if (data.val != VPF_YAPF) {
+				data.output = SPR_WARNING_SIGN;
+				return true;
+			}
+			return false;
+
+		case SOGCT_GUI_WARNING_TEXT:
+			if (data.val != VPF_YAPF) {
+				data.text = STR_CONFIG_SETTING_ADVISED_LEAVE_DEFAULT;
+				return true;
+			}
+			return false;
+
+		default:
+			return false;
+	}
+}
+
 /* End - GUI callbacks */
 
 /**

src/table/settings/pathfinding_settings.ini

@@ -11,6 +11,8 @@
 static void InvalidateShipPathCache(int32_t new_value);
 static void CheckYapfRailSignalPenalties(int32_t new_value);
 
+static bool TrainPathfinderSettingGUI(SettingOnGuiCtrlData &data);
+
 static const SettingTable _pathfinding_settings{
 [post-amble]
 };
@@ -115,6 +117,7 @@ str      = STR_CONFIG_SETTING_PATHFINDER_FOR_TRAINS
 strhelp  = STR_CONFIG_SETTING_PATHFINDER_FOR_TRAINS_HELPTEXT
 strval   = STR_CONFIG_SETTING_PATHFINDER_NPF
 cat      = SC_EXPERT
+guiproc  = TrainPathfinderSettingGUI
 
 [SDT_VAR]
 var      = pf.pathfinder_for_roadvehs