Crossing tunnels is a cheat which allows you to build tunnels that intersect underground. Vehicles inside the tunnels can pass through each other like ghosts instead of colliding. Manual does not check for collisions underground.
Cheat or not a cheat?
Developer eis_os originally implemented this feature in TTDPatch. He thought that long tunnels with entrances on the same elevation should be able to cross over or under each other underground. Unfortunately this feature is not limited to long tunnels; it allows unrealistic crossing of 1-tile-long tunnels.
OpenTTD has not implemented crossing tunnels as standard, only as a cheat.
Using the cheat
Press+ + and select "Enable tunnel crossings".