In 124.3.1, tram track can be laid on monorail, narrow gauge, etc. However, tram cannot be on tunnels of them.
Furthermore, tram cannot connect to track under the ground.
So, I make tram can be on monorail, narrow gauge and maglev tunnels, and tram in tunnel can connect to track tunnels.tram_connect_to_track_in_tunnel.patch
tram_in_tunnel.png
Figure: track, road, and monorail tunnels with tram track. tram can branch from track tunnel, and they can change lines as long as there are other tunnels.