I have an idea for a steam loco sent to the scrap yard in 1964 and later found in the 1990s under a pile of junk. Restored and put back into service on a preserved steam line. Wearing a new/retro style livery.
What do you think of this?

But to tweak it a bit. How about during one of the oil shortages of the period a still nationalised B.R. bring it back into service to run on one of the cross-country routes ? You could then have them trawling museums and preservation societies for more of the lovely old things 
That is one of my ideas - but the kit I'd use for that is half done already.
I'm thinking of something a little different, the kit arrives next week.

Have you read John Gardner's
Golgotha? It's set in a 1970s where the USSR has taken all of Europe and Britain surrendered. The oil crisis leads to steam trains making a comeback.