A bit of an enigma this one, nobody seems to know exactly what it is other than extremely large.
This is a four cylinder Pacific, I'm pretty certain somebody's experimental project featuring 135 degree cranks for the inside cylinders, requiring two sets of Holcroft conjugated valve gear to work their valves, with the 2 to 1 levers intricately working their way around each other. Nicely executed, although not something I've ever seen done in full size - I'm sure somebody will let me know different (probably within about an hour of listing this).
Work to date appears to be by an epxerienced hand, both in terms of design and execution. It runs on air, has a largely complete tender featuring the same vacuum brake arrangement as the locomotive as well as twin water pumps and a tank containing a large hand pump.
The rear pony truck is detachable and obviously set up for a wide firebox boiler. When complete it is going to be a massive great thing, nearly seven feet long with its tender.
Locomotive 57 inches
Tender 24 inches