I seem to recall that the "Steam Railway" article included the quoted words of a chap who claimed to have, I think in 1969, seen in an out-of-the-way section of Shrewsbury's rail installations, rows of steam locos, cold and with boarded-up cabs, "parked" on sidings. (He had, I think, fallen asleep during his rail journey from Aberystwyth: woke up when his dmu was, after terminating at Shrewsbury, on its way to wherever in the rail complex it was to be serviced -- at the station, nobody had noticed him asleep, and got him out on to the platform. He awoke, adjacent to all these dead "steamers".) The guy wasn't a railway enthusiast; but his occupational field was mechanical engineering -- he was thus unlikely to misconstrue as steam locomotives, "something which was altogether something else". I believe SR quoted this as the best account which is known, of someone seeing first-hand, what could be elements of the strategic steam reserve. Of course -- as per
@Spartacus's Post #12 -- these locos could simply have been in transit for scrapping.