diff --git a/src/rail_cmd.cpp b/src/rail_cmd.cpp
--- a/src/rail_cmd.cpp
+++ b/src/rail_cmd.cpp
@@ -329,7 +329,7 @@ Foundation GetRailFoundation(Slope tileh
 static CommandCost CheckRailSlope(Slope tileh, TrackBits rail_bits, TrackBits existing, TileIndex tile)
 {
 	/* don't allow building on the lower side of a coast */
-	if (IsTileType(tile, MP_WATER) || (IsTileType(tile, MP_RAILWAY) && (GetRailGroundType(tile) == RAIL_GROUND_WATER))) {
+	if (GetFloodingBehaviour(tile) != FLOOD_NONE) {
 		if (!IsSteepSlope(tileh) && ((~_valid_tracks_on_leveled_foundation[tileh] & (rail_bits | existing)) != 0)) return_cmd_error(STR_ERROR_CAN_T_BUILD_ON_WATER);
 	}