Sold October 2020 Back to Archive

I first met the builder of this engine nearly twenty years ago - he was selling a "King Arthur" at the time to make room for the 5 inch gauge "Duchess" that was already well-advanced in his workshop.

Some time after finishing the "Duchess", the opportunity arose to acquire a commercially built boiler and one of three sets of castings for a 3 1/2 inch gauge 4-6-6-4 Union Pacific "Challenger", produced in the 1970s by Al Duzey of New Jersey.

A lifelong fan of American locomotives, the chance was too good to miss - the parts were shipped over from the USA whereupon work commenced on building as accurate a model as he could, with reference to works drawings.

The pictures opposite show the level of detail attained - it includes prototypically correct cast iron frames; four Nathan DV5 lubricators (the only working versions we've seen in this scale), investment cast from 3D printed waxes; fully working compensated springing and suspension; front truck lateral control by geared roller and inclined planes and all driving axles running on roller bearings, with ball bearings to all other axles and valve gear.

The locomotive is fitted with scale rigging for the compressed air brakes.

Working swivel joints had to be designed and manufactured for the steam and exhaust pipes, which the builder found to be reliable.

Work continued on the locomotive until 2016 when, unpainted, it had its first steam test. Subsequent steamings were used to iron out the inevitable teething problems that arise with such a large and complex piece of machinery.

In January 2018, close to completion it was it exhibited at the London Model Engineering Exhibition, before entering public service at the builder’s club for the first time later that year.

The boiler is oil-fired, with paraffin stored in the tender. The subject of much development work, initial problems centred around the change in pressure at the burner as the fuel tank level decreased during running. A small electric pump and separate tank were installed to create a constant head feed, curing the problem. The engine is fitted with a compressed air coupling for the fuel atomizer, used when starting from cold, before the steam supply takes over for running.

Fitted with a toughened glass firehole door, the flame colour can be monitored, with fuel control from a valve under the footplate. There is an electric igniter in the bottom of the firebox, used for restarting when hot it produces spectacular bangs and pyrotechnics if the engine flames out under load and is not re-lit promptly!

From the start, the builder intended the engine to look as realistic as possible when running. To that end - given that he wanted to run with the cab roof on, and the length of the fourteen wheel "Centipede" tender would require the arms of a gibbon to reach the cab controls - an ambitious "fly by wire" control console was devised and built for all controls bar throttle and fuel supply, which retain a mechanical linkage for safety reasons.

Model aircraft servos have been used, fully wired rather than radio controlled. Whilst they proved to be reliable in the tender, the four steam valve controls in the cab have proved problematic, initially failing very quickly through overheating.

The Mk2 system mounted the four servos in a block of resin to insulate them, which delayed their failure - but fail they still did.

Mk3 moved the block to the back of the cab, mounted on a purpose-built thin copper radiator supplied with cooling water by an electric pump in the tender. The last entry in the builder’s diary reported the results of running with this, with intended modifications for the future.

Steamed on a handful of occasions, the locomotive remains very much an experimental project - it has benefited from the experience and ability of a highly resourceful builder and will need a new owner with a similar mindset. It's less an engine, more a way of life!

Possibly unique in this gauge, the whereabouts of the other two sets of castings made before the patterns were destroyed over forty years ago are currently unknown.

Complete with a hydraulic work/storage stand with built in rolling road, toolkit, spares and builder's notes and photographs. There's a short video of it running on its rolling road here.