[Brakenshire]

Don't you need wide tunnels to fit normal trains, let alone high speed trains, because they operate on a higher gauge? I know in London the new Crossrail tunnels have been deliberately made a lot wider than the existing Underground tunnels, because that allows normal trains to run through it. The Underground trains are narrow-gauge light-rail.

[JoeSmithson]

Not just Crossrail, even the new tube lines, such as the Northern Line Extension to Battersea are dug much wider than the old tunnels they connect up to. Obviously they carry the same trains as the rest of the line.

The narrowness of the old tunnels is a constant hurdle to improving the tube.

[KJKingJ]

Indeed, wider tunnels make maintenance much easier, and also allow evacuation of the train via the regular side doors rather than through the cab door.

[Symbiote]

> The Underground trains are narrow-gauge light-rail.

No, they're standard gauge heavy rail; in some places the track is shared with ordinary passenger trains. The older tunnels just have very tight clearance.

You may mean the structure gauge, which is the clearance for tunnels, platforms and so on. That is normal-rail for Crossrail.

[spiralx]

You need wide tunnels to reduce aerodynamic drag on the trains.