Long Tunnels

The Long Tunnels connect The Great Gate with the Dwarven Catacombs just north of Undertown. There are two completely separate tunnels, very straight, with two smooth rail tracks in each. The tunnels slope downwards for several miles to pass under The River before sloping back up towards The Great Gate.
Being a vital supply route, the Long Tunnels are well maintained by dwarven engineers and accidents are rare, even though the railcars reach hair-raising speeds in pitch darkness powered mostly by gravity. When the cars slow near their destination, the operator stops them, signals ahead with lights and a horn blast, and a small car attached by a winch is sent down to couple with the front car and pull it up the rest of the way.
The two Long Tunnels debark on opposite sides of The Great Gate. Most people find traveling via the Long Tunnels to be frightening, and if they need to travel to the Great Gate, they go through The Marsh instead.