Wikipedia: Strategic steam reserve

A Strategic steam reserve (SSR) is a collection of withdrawn steam locomotives which is kept in working order for possible use in a national emergency. During the Cold War several countries, including Sweden and the Soviet Union, kept SSRs and the United Kingdom is reported to have done so, albeit without hard evidence.

Many European railways have a large mileage which is electrified. In the event of a war, or major natural disaster, electricity supplies could be disrupted and electric locomotives would be unworkable. Diesel locomotives could also be at risk for two reasons:

– supplies of imported oil might be cut off,
– solid state components in diesel locomotives might be destroyed by the electromagnetic pulse (EMP) from a nuclear weapon.

The alleged British ‘Strategic Reserve’ is a potent and frequently recurring urban myth amongst railway enthusiasts. Following the complete withdrawal of mainline steam traction in 1968, the myth persisted for decades that a reserve of locomotives had been retained for such a strategic purpose. As no official reserve existed, nor was any reserve obviously visible, this encouraged many fanciful explanations for where the reserve might be hidden, usually in some tunnel or mine. One theory even claimed that the growing steam preservation movement was itself the Strategic Reserve.