Sir Nigel Gresley’s 1929 built No. 10000 was not just the only 'Baltic' tender locomotive ever to run in Britain, it was also a four-cylinder compound with a 450 psi water-tube boiler, which made it trebly unique. It is generally considered as a failure in this condition, and was rebuilt after 6 years with a conventional fire tube boiler, and streamlined, becoming, in effect the only 4-6-4 of the A4 class. But before this rebuild the engine had handled The Flying Scotsman and other prestige trains, so was it really a failure? This is the question the author tries to answer here, aided by the discovery of a large file on the locomotive in the NRM, and his suggestions of what might have been are fascinating. That said, and whilst not disputing his conclusions, I am not entirely happy with his methodology, and he has also taken some remarks in Andre Chapelon's La Locomotive a Vapeur at face value which, in this instance, is perhaps optimistic.  But still an interesting read by any standards. Quite a bit of technical meat, and numerous B&W photographs, but only outline drawings. Harbound. Kestrel Railway Books