When the D40 was first introduced, it was based on the D36 steam roller with only the bare minimum of modification. That version can be seen here and is sometimes referred to as the Mk1. The Mk1 was most unsatisfactory, with the front end looking extremely unrealistic. The rest of the engine remained identical to the roller except for slightly narrower rear wheels, a longer canopy and a colour change. They later redesigned the front end to look more like a traction engine (Mk2). Confusingly, the official model number remained the same so that Mk1 and Mk2 D40s look very different.This is the later (Mk2) version. The burner is a standard Wilesco Esbit fuelled type fitted under the rear end of the boiler. The boiler is approx 165mm long x 45mm diameter. It's not easy to measure because it's enclosed in an outer covering. At the rear end is a typical Wilesco round water level sight glass; somewhat obscured by the dummy bunker, which is not removable. Also fitted are safety valve, throttle and whistle, all pretty standard Wilesco style.
The steam pipe comes out of the top of the throttle, round a loop to the under side of the valve chest. The valve is the usual Wilesco type, sometimes called a "Piston" valve because it's round but is not a true piston valve, closer to a slide valve but round in shape. A drip feed oiler is fitted on the top of the chest. The valve controls steam into a double acting cylinder, and is itself controlled by a slip eccentric on the crank shaft.
The piston is 13mm diameter (assuming it's the same as the D16, which is a good bet). Stroke is measurable at 16mm. The main piston rod is connected to the crank via a con rod with no cross head. The crank and slip eccentric are together at one end of the crank shaft. At the other end is a 78mm diameter mazac flywheel. Exhaust steam is fed from the valve chest into the smoke box to be exhausted through the chimney.
Drive from the crank shaft to one rear wheel is via a two stage gear chain, unfortunately using thin pressed steel gears which sound very "tinny" while running. A lever shifts the final gear in and out of mesh to allow the engine to idle freely. The wheel spokes are disappointingly tinny in appearance. This applies, also, to the front wheels but is less obvious there.
The front wheels are steered by chains wound round a spiral shaft which is turned by a contrate gear and a spur gear attached to the steering wheel. The engine is covered by a full length canopy.