To put a bit more meat on the bones about the Decapod I've dug out The Great Eastern Railway by Cecil J Allen.
The Decapod had "a grate area of 42 sq.feet and the 5' 3" diameter boiler was pitched with its centre at 9' above rail level, provided a total heating surface of 3,100 sq.feet. With his three cylinders of 18 1/2 inch diameter by 24 inch stroke, Holden had to solve another problem caused by the position of the middle cylinder in line with the two outside cylinders. This he did in a most ingenious way by forking his middle connecting rod so that it completely enclosed the leading coupled axle, which was cranked by 3 1/2 inches in order to synchronise its revolution with the rod. So far as I know, this unique arrangement, which Holden patented, has never been used on any other locomotive"
The testing was carried out at "a special plant near Chadwell Heath to give an electrical recording of the actual rate of acceleration. The tests took place from February to June 1903 when no.20 was run between Stratford and Brentwood, and at last after priming and other difficulties had been overcome, her designer had the supreme satisfaction of seeing his creation move a load of 18 four-wheel coaches , loaded with pig iron to represent passengers and with a total weight of 335 tons, from a dead start up to he prescribed 30mph speed in just under the specified 30 seconds ".
As to why further examples were not built "for an engine of such power, the supplies carried were relatively small - 2 tons of coal, and 1,300 gallons of water, distributed between a well tank under the bunker and a second tank between the frames ahead of the firebox - but increased capacity would have meant more weight and that was not considered feasible. .... But as suburban traffic at that time was not showing any signs of increase, the Great Eastern directorate saw little reason for all the bridge strengthening that would have been necessary to carry a locomotive of this weight, and after a lapse of 4 years, it was decided to convert no.20 into something useful."
Best regards
Simon