Basil Jet wrote:

I've always wondered... do the wheel lathes merely subtract metal, so a
wheel with a three millimetre dent comes out losing three millimetres of
radius, or do they pour some liquid metal on the dent, wait for it to
cool and then lathe it down to the same radius as the rest of the wheel?

I'm pretty sure they just skim material off. You couldn't add an alloy with
the right characteristics. It would also damage the wheel's heat treatment
if you poured liquid steel on to it.

Incidentally, it's a flat, not a dent. In effect, a section of the circular
profile has been planed off when the locked wheel slid along the track.