I don't think they can be rebuilt, at least I have never seen any claim that they could, and I have used Konis on several cars over the years. They really do last about 5 times as long as Woodhead Monroes, the cheap, nasty brand used by several car manufacturers in the 1970s, and probably even now, but they wear out eventually, like anything with moving parts.
Actually, very few brands of damper can be rebuilt, most are sealed, for example by swaging or crimping the end.
Edit: I have found a rebuildable Koni damper, but you did say it was for a Mini, not a train:
http://www.koni-enid...ebuildable.html
Another edit: Some AVO and Bilstein dampers are rebuildable, but these are not brands which seem to be commonly used on Minis.
Yet another edit: Protech too.
Edited by tiger99, 02 February 2014 - 08:05 PM.