This documentary gives the background to Mallard's record-breaking run, with lots of 1930s footage of the A4s and their LMS rivals, the Duchesses.

Gresley was aiming for a speed of 130 mph in 1939, but the war broke out before the attempt could be made.

The A4s looked as if they had been designed with their streamlining, whereas the streamlining on the LMS "Coronation" class looked like a brutalist afterthought (which to an extent it was), which was later removed.

Both classes looked better than their Chapelonisé counterparts across the Channel, which were marred by pipes, rods, wires and all sorts of excrescence all over the boiler. They were nevertheless more powerful and economic than anything built in the UK.

My favourite for looks: the humble LMS "Black 5". All Stanier's engines looked well-proportioned, but the "Black 5"s could be coupled to the front of anything from a London-Crewe express to a coal train. They were built in hundreds.