Interesting that people suggest locos like 8F or 9F being the obvious choice. In fact I would suspect that smaller, simpler locomotives like the Austerity Tank would be the most obvious choices - much easier to run and maintain in a post-apoc environment.
In theory the idea of an SSP is a good one, in the event of a small nuclear exchange (ie. one as imagined in the 1950s/60s, before the ideas of Mutually Assured Destruction), in the months following, Britain would probably need to rely on locally available materials such as coal, rather than imports of oil which might well dry up.
Obviously the railways would not be useful until they had been assessed for damage to bridges etc. but after then running a freight only service, supplying coal power stations and moving food supplied for example, would be essential to get the country moving and could be achieved without too much difficulty with some basic organisation.
One of the most commonly touted issues with the use of steam trains here is the removal of infrastructure (ie. water towers) after 1968, but this would probably be a quite minor factor considering the amount of other rebuilding work required after a nuclear strike.
However, although it might have been a good idea I find it highly unlikely that we did actually have a reserve. To be effective there would have to have been dozens of stores around the country (the idea that all the locos were stored in a single location eg. Box Tunnel would make no sense at all) with a total of hundreds of locomotives. With the Cold War long gone and old secret nuclear bunkers now open to the public, there is no reason at all why the government would continue to pay out for the storage and maintenance of these locomotives, nor any reason why they would keep their existence a secret.
