The Sir Nigel Gresley, named after the designer of the engine. This is one of the last few surviving A4s, running on the North Yorkshir Moors Railway.  The A4s were operated by LNER, London North Eastern Railway between London and Edinburgh, easily recognisable by their distinctive streamlining.  The most famous of these enignes is the "Mallard" (on dispalay in the National Railway Museum, York) which still holds the world record for steam trains which it set in 1938.

More pictures from the NYMR can be found here in my flickr set