5 inch narrow gauge Decauville Mallet 0-4-4-0T

The Exposition Universelle of 1889 in Paris was the fourth of eight exhibitions held between 1855 and 1937, perhaps most famous for the creation of the Eiffel Tower - intended as the time as a temporary structure!

Spread over a large site, Decauville were contracted to build a 600mm gauge railway, a little under two miles long, to carry visitors around the exhibition.

The locomotives and rolling stock were supplied by several different companies, with Decauville themselves building a number of 0-4-0 + 0-4-0 Mallet compounds, their flexible wheelbases allowing operation on curves down to 20 metre radius.

After the exhibition the railway was dismantled, the locomotives dispersed to various other lines throughout the world - the articulated wheelbase allowing then, as now, the use of a larger, more powerful locomotive on tight radius curves than could be achieved with a rigid wheelbase.

I first saw this model some years ago when we were exhibiting at the Midlands Model Engineering Exhibition. A friend who was visiting recommended I take a look at the model, he enthused about its technical interest and engineering virtuosity. Constructed to exhibition standard by a highly experienced engineer, the model was being built at 2 1/2 inch scale using his own drawings derived from a works general arrangement.

Work to date is to an exceptional standard throughout - one of those models that, the longer you look at it the more you see. The eighty high resolution pictures shown opposite convey something of the quality, complexity and fidelity to prototype of the build.

The builder served as a club boiler inspector, his work on the boiler, as with everything else about the engine, first rate. Tested off the frames when new, since arriving here it's had a twice working pressure hydraulic test showing it to be exactly as expected - thoroughly sound, finely made with not a sign of leaking or deflection of plates at 220psi.

The engine runs beautifully on air with dead square, well-defined exhaust beats - all the more creditable given that it's a compound, with the low pressure cylinder designed to extract energy from still expanding HP exhaust steam, rather than spent compressed air. The HP bogie runs faster for that reason - in steam the work would be more evenly distributed between the two.

We couldn't resist setting up a pair of rolling roads as if on a tight curve to see it running fully articulated - you can see that in the second half of the video on our YouTube channel here.

We would put the engine at better than 90% complete, although don't underestimate the time required to finish the model to the daunting standard the late builder has set. Probably unique in this gauge - possibly as a live steam model in any gauge - it has the potential to make a very fine model indeed.

The engine comes complete with a large quantity of drawings and copies of articles comprising the builder's research and development of his design for the 5 inch gauge model.