(svn r21841) -Feature: [NewGRF] Allow to define other railtypes that should be introduced if a particular rail type is introduced, e.g. to make sure slow rail is introduced when fast rail gets introduced

src/rail.h

@@ -216,6 +216,11 @@ struct RailtypeInfo {
 	 */
 	byte map_colour;
 
+	/**
+	 * Bitmask of which other railtypes are introduced when this railtype is introduced.
+	 */
+	RailTypes introduces_railtypes;
+
 	/**
 	 * Sprite groups for resolving sprites
 	 */