How is this supposed to work? Other than crash and show "No path" I can't get anything to work.

Short: Build rails, place one or two tunnels, take train into tunnel and connect tunnel entrances.

It's a bit "no userfriendly" at the moment, and also very fragile. As soon as it is stable enough I will try to give it the same features as mods like pipelayer and beltlayer, including blueprint support.

Also, placement should be done on segments without signals. Otherwise you might not be able to place the outgoing tunnel part.

Just a small suggestion on the building part, what about a second method that works like the underground belts basically you place it like you would place a underground belts and the rails on the other 'tunnel world' connects automatically? So it would be a second item which you could call like 'simple tunnel' just to make simple one-way tunnels.

Something like that is on the todo. If you are familiar with the pipe layer or belt layer mods, that what I was thinking to do.

Hello I've problem to set Train Tunnels.
Every time I want to put a tunnel, rails appear there instead of the tunnel. It doesn't matter whether it is horizontal or vertical, nothing happens except that the rails are simply placed instead of a tunnel.

With Shift + Tunnel he sets the tunnel but you can't use it as if he doesn't want to go underground.