charles wrote:

whisky-dave wrote:

even the underground trains aren't standardised are they.
You have specific ones for specific lines.

I think the track gauge is the same - it's the tunnel height that differs.

Not simply the height, the loading gauge defines the profile (e.g.
curved, curved with corner notches, or flat) as well as the dimensions.

https://en.wikipedia.org/wiki/Loading_gauge#Great_Britain