More conspiracy theories...
Rumours of the Strategic Reserve suggest it could also include some diesel locomotives, which in deep bunkers would be protected from the electromagnetic pulse of a nuclear attack. A possible source for diesel locos could have been a secret add-on to the Cuban order for class 47s, which was already kept quiet to avoid Brush's parent company Hawker Siddeley losing US military contracts for exporting to the embargoed communist nation.
With much of the country uninhabitable due to deadly levels of radioactivity, a "47" hauls a train of refugees towards the undamaged ports on the west coast of Wales, where they hope to gain passage on a ship to one of the areas of the world which escaped the war, like Australia, South America, or Africa. The loco bears the logo of the Royal Logistics Corps, whose members are reported to still train on steam locos at preserved lines on a regular basis. Two of the tracks have had their rails stripped to repair other damaged sections of the line.