One of the most famous being 466036 at Slade Green. In the end I paid a visit (12/08/96), and was very pleasantly surprised to be taken to see it - fortunately no tracks to cross.
Yes there are erm 50% extra rails that you do not want to step on !. I think I heard a few people were waiting for a 466, I must have been lucky.
Obvious question - why not obtain the spare parts from the manufacturer? Particularly as the trains are brand new!
When it comes to major parts (engines and bogies for instance) that are changeable but not likely to need replacing (just refurbishing) I suspect there are only ever just enough. If a bogie or engine has been written off then there is not another one on the shelf. There will be quite a lead time getting things like these. I suspect there might have been more than one spare engine but they might have got through them all !. All conjecture on my part but based on sense and experience in other fields.
I would imagine they will not have bought expensive spares that they believed were unlikely to be necessary. Compounded by the fact that they have a surplus of 755s means no urgency. Just occurred to me, suppose a bogie has been robbed to keep a 745 going - severe wheel flats anybody !.