Thanks very much for all the helpful comments. Much appreciated.
Some progress to report - I took courage in both hands, and removed one pair of wheels from a Collett coach. There was a small ball of fur wrapped around each bearing. I have now cleaned out the bearing cups, wiped off the unnecessary oil, reassembled, and repeated the process for each coach. They're definitely running much better. I have also issued a written warning to the cat, so there shouldn't be any repetition of the problem. There's definitely some hair in the bars connecting the wheels on the loco, so that's the next task. Once I have done that, what's a suitable cleaner for the wheels and rails?