StephensonÕs Eclipse, 1832
The original plans for the Eclipse, undated but attributable to late 1828 or early 1829, show an 0‑6‑0 with twin vertical boilers. It was to have plateway wheels 3ft in diameter and set at 3ft 1½in inside gauge for running on rails of 3ft gauge, steeply inclined cylinders and valve motion driven by gears on the rear axle and by rocker arms. Note: Plateway wheels were designed to run on a flat plate – not rails. But the design was drastically changed, for an outline drawing dated July 1829 shows a horizontal boiler 2ft 9in by 7ft 2¾in, and the Engine Description Book confirms that as built the engine had a single non-return elliptical flue (this being just before the ROCKET and the multitubular boiler). The chimney was no doubt bent up through 90 degrees from the front of the flue. The cylinders, 7in by 20in, were inclined and mounted on the boiler back; the frames were of flat bar 3in by 1in, and the wheels were 3ft. The Penydarren engine, in a packing case, was dispatched by sea from Newcastle on 18th July 1829
The engine, apparently nameless, hauled trams on the internal Penydarren system for two years or so before being returned to Stephensons in 1832 for conversion to 4ft 6in gauge — the Penydarren Tramroad gauge plus 2in play. The boiler shell was retained but given 82 copper tubes, a smokebox and a vaulted firebox with dome on top; the heating surface now totaled 289.75 sq ft (so the Description Book, but the drawings show only 60 tubes). The cylinders remained in the same place, but new axles were needed with the cranks inside the wheels, and a new sandwich frame, typical of Stephenson practice of the period, was fitted outside the wheels. The arrangement was altered to 0‑4‑0, with new wheels of 3ft 6in diameter (according to the Particulars Book; 3ft 2¾in or 3ft according to the drawings; 3ft 4in according to The Cambrian). The chimney stood 12ft 7in high above rail level, and though the drawing shows it fixed, it must have been lowerable to pass through Plymouth tunnel. The engine that resulted from this metamorphosis was in general very like the standard four-coupled Stephenson 'Planet' type of the time, except for the inclined cylinders which were old-fashioned by then.
In this new guise the engine began to work on the Penydarren Tramroad on 22nd June 1832.
This information is excerpted from The Industrial Railway Record where more information on the Eclpise can be found.