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So when we had a call from a man with a part-built "Big Boy" recently - and one that came with an interesting story to boot - naturally we couldn't resist going to have a look at it.

By 1941 Union Pacific had found that the 4-6-6-4 Challengers - introduced in 1936 - required double heading over the route between Ogden and Cheyenne when hauling 3,600 ton trains. In conjunction with the American Locomotive Company, they redesigned the Challenger from the ground up, with the aim of producing a single locomotive capable of hauling these heavy trains.

With enlarged firebox, the new boiler's working pressure was increased to 300psi. Four extra driving wheels were added - of slightly smaller diameter than the Challenger's, increasing tractive effort whilst maintaining an axle loading of just over thirty tons.

The new locomotive was of massive proportions, contributing to a high degree of reliability; designed to ride safely at 80mph, tractive effort was measured at over 135,000lbf - something over three times that of an LMS Duchess.

Officially designated the "Wasatch" class after the range of mountains through which the Union Pacific ran, legend has it that a fitter in the works scribbled "Big Boy" in chalk on the smokebox door of the first engine as it neared completion - the name by which the world has known these engines ever since.

25 locomotives were built in total between 1941 and 1944, proving immediately successful. Eight have survived in preservation with one, No.4014, acquired and restored by the Union Pacific between 2014 and 2019 to celebrate their 150th anniversary.

The resulting locomotive looks and sounds absolutely superb, a truly awe-inspiring piece of machinery (and, for those who haven't yet seen it in action, there's no better video of it than this one, superbly shot in high definition, which we must have watched a hundred times since it was posted back in 2021!)

This 3 1/2 inch gauge, 3/4 inch scale model is the work of one man, a Mr Jones of Cheltenham, about whom personally we know little other than by his work and the notes and drawings he left. Possibly a professional engineer - his notebook and some of the correspondence is addressed to him at "Metalways Experimental & Production Engineers" - the project was started in 1947, only four years after the last full-size locomotive of the class was built. He meticulously recorded the hours worked and costs incurred every year until 1982 when - with 3,727 hours logged in his diary, the last entry states poignantly "work stopped for op"...

The project then languished until 2012 when it was acquired by a highly capable designer and engineer who set it up on a length of track in his living room, intending it as a project for some future date. In the meantime he carried on building other things, as well as producing a highly detailed design for the Liverpool & Manchester "Lion", based on his research and measurement of the original locomotive.

By the time his bench was clear and opportunity to resume work on the Big Boy presented itself, he'd found that - by now in his ninth decade - the engine looked a rather more daunting size and so, reluctantly, he decided it was time for a new owner to carry on with it.

Work to date is to a fine standard - it's a highly detailed build, standard of fit and finish is good throughout. The chassis both turn over freely without tight spots and would appear to require little work to get them running on air.

The tender is largely complete - where with British prototypes we're always impressed by builders who fit a water scoop, this one takes it to a new level, with a screw auger in the bottom intended for the mechanical stoker!

There are few published designs for these engines in any scale, and we're pretty sure none that came out as early as 1947. One can only surmise that it was originally drawn up with reference to published full-size details - there are tantalising snippets of the builder's notes and drawings, as well as some obviously commercially produced drawings, including one for the boiler. Copies of some of these are reproduced on a supplementary page here (because, like most things to do with this engine, they're too big to fit on the main listing page...)

In later years True to Scale Services of Bromley, Kent apparently produced drawings for a Big Boy in 3 1/2 inch gauge, but finding completed models to that - or any other - design is hard going. We suspect that fewer Big Boys have been modelled - in this gauge or larger - than were ever built in full size.

So, a rare opportunity to complete an engine which is likely to create a good deal of interest wherever it appears. A large, complex locomotive at ten feet long, it currently weighs in at close to four hundredweight - when complete we estimate that it will tip the scales at around the same as a pair of 5 inch gauge 9Fs.

We'd put the project at about 70% complete. Work to date comprises:

• Complete pair of chassis complete with lubricators
• Largely complete articulated steam pipework to cylinders
• Part-finished platework for cab, running boards
• One steam pump (a pair were fitted above the front bogie
• Seamless boiler barrel cut at rear to form outer wrapper water legs, combustion chamber, main and superheater flue material
• Quantity of boiler and backhead fittings (some in a fabulous vintage Stuart Turner box, with 10 1/2d postage on it!)
• Largely complete 14 wheel tender

There is a quantity of notes, drawings and a build diary giving hours logged and expenditure, dating back to 1947; copies of works drawings including a general arrangement and some more recent - apparently commercially produced - drawings, including one for a copper boiler in this scale.

In the first instance the engine needs stripping completely and cleaning - it was last worked on over forty years ago and looks to have spent at least half that time on the late builder's bench, getting grubbier as time passed. However, underneath the grime the brightwork remains in good condition, it's covered in oil and dust rather than suffering from corrosion.