The Tunnel command is used to hide selected (Select) tracks (as in a tunnel).

Select the tracks you want to hide in a tunnel. Selecting a hidden track 'reveals' it.

A tunnel portal is drawn where a hidden track connects with an 'unhidden' track. You can split (Split Track) a track to place a tunnel portal at a particular spot.

The Draw Tunnels Radio button group on the Options>Display dialog (Display Dialog) controls whether hidden tracks are not drawn, drawn as dashed lines or drawn as normal lines.