diff -p -up ./src/rail_cmd.cpp.CVE-2012-3436 ./src/rail_cmd.cpp
--- ./src/rail_cmd.cpp.CVE-2012-3436	2011-02-07 22:37:22.000000000 +0000
+++ ./src/rail_cmd.cpp	2012-08-07 20:24:36.830989644 +0000
@@ -2527,7 +2527,7 @@ set_ground:
 static TrackStatus GetTileTrackStatus_Track(TileIndex tile, TransportType mode, uint sub_mode, DiagDirection side)
 {
 	/* Case of half tile slope with water. */
-	if (mode == TRANSPORT_WATER && IsPlainRail(tile) && GetRailGroundType(tile) == RAIL_GROUND_WATER) {
+	if (mode == TRANSPORT_WATER && IsPlainRail(tile) && GetRailGroundType(tile) == RAIL_GROUND_WATER && IsSlopeWithOneCornerRaised(GetTileSlope(tile, NULL))) {
 		TrackBits tb = GetTrackBits(tile);
 		switch (tb) {
 			default: NOT_REACHED();
@@ -2831,6 +2831,14 @@ static CommandCost TestAutoslopeOnRailTi
 	return  cost;
 }
 
+/**
+ * Test-procedure for HasVehicleOnPos to check for a ship.
+ */
+static Vehicle *EnsureNoShipProc(Vehicle *v, void *data)
+{
+	return v->type == VEH_SHIP ? v : NULL;
+}
+
 static CommandCost TerraformTile_Track(TileIndex tile, DoCommandFlag flags, uint z_new, Slope tileh_new)
 {
 	uint z_old;
@@ -2840,6 +2848,9 @@ static CommandCost TerraformTile_Track(T
 		/* Is there flat water on the lower halftile that must be cleared expensively? */
 		bool was_water = (GetRailGroundType(tile) == RAIL_GROUND_WATER && IsSlopeWithOneCornerRaised(tileh_old));
 
+		/* Allow clearing the water only if there is no ship */
+		if (was_water && HasVehicleOnPos(tile, NULL, &EnsureNoShipProc)) return_cmd_error(STR_ERROR_SHIP_IN_THE_WAY);
+
 		/* First test autoslope. However if it succeeds we still have to test the rest, because non-autoslope terraforming is cheaper. */
 		CommandCost autoslope_result = TestAutoslopeOnRailTile(tile, flags, z_old, tileh_old, z_new, tileh_new, rail_bits);