diff --git a/yapf/yapf_rail.cpp b/yapf/yapf_rail.cpp
--- a/yapf/yapf_rail.cpp
+++ b/yapf/yapf_rail.cpp
@@ -260,6 +260,7 @@ bool YapfFindNearestRailDepotTwoWay(Vehi
 	return ret;
 }
 
+/** if any track changes, this counter is incremented - that will invalidate segment cost cache */
 int CSegmentCostCacheBase::s_rail_change_counter = 0;
 
 void YapfNotifyTrackLayoutChange(TileIndex tile, Track track) {CSegmentCostCacheBase::NotifyTrackLayoutChange(tile, track);}