Use new VoxelManip cleanup API

mods/carts/functions.lua

@@ -41,15 +41,9 @@ end
 function carts:is_rail(pos, railtype)
 	local node = minetest.get_node(pos).name
 	if node == "ignore" then
-		local vm = minetest.get_voxel_manip()
-		local emin, emax = vm:read_from_map(pos, pos)
-		local area = VoxelArea:new{
-			MinEdge = emin,
-			MaxEdge = emax,
-		}
-		local data = vm:get_data()
-		local vi = area:indexp(pos)
-		node = minetest.get_name_from_content_id(data[vi])
+		-- we really need to know, so load it
+		minetest.load_area(pos)
+		node = minetest.get_node(pos).name
 	end
 	if minetest.get_item_group(node, "rail") == 0 then
 		return false