A useful tool is the "dighole", where you take subway tunnels below the terrain, and avoid the ground textures from interfering with the trackside.
The London Underground tunnels "LUL tunnel" for tube trains is good but only for the tube 1967 and 1996 stock. LUL tunnel junction is good, as you can construct wide tunnels, providing you construct the track in each direction carefully. This assett provides one wall and a ceiling, so joining two together makes nice wide tunnels.
