I suppose if you count the bodies of the trains the HSTs and what not must be on millions of miles each. 142s i believe are up to 4 million miles and counting.
The thing is, the running gear will have all have been changed long before that. If you notice on some of the 15X DMU stock you can see ATL 500,000 on the blocks for 500K mile engine rebuilds. Not that bad really considering!!