It could potentially relieve pressure on the busy underground section of the network.

Line 4 is the only one in the system that does not have underground sections.

All but one of the lines run together in a single underground section through the city centre, and then go their separate ways at either end.

There is also an underground section (with an art installation).

In underground sections the rails will be 15m below ground level.

Under the circumstances, it was too much to hope they would be able to penetrate the underground section of Settlertown again.

There are also two vacuum trains that clean underground sections of track.

The command headquarters was located in an underground section of the cave.

I wondered about the underground section of the elevator shaft.

For visitors, the single-file walk along the underground section of the wall takes about 15 minutes.