modified version of the previous engine. This engine was equipped with direct injection and the bore diameter was enlarged to 120 mm. The stroke remaining at 142 mm. This engine produced a lower brake power output of 105 hp at 1,650 rpm. An unusual feature was the external cardan shaft drive from the gearbox on the rear of a horizontally mounted engine to road-vehicle style reduction boxes outboard of the two axles on one bogie. Later units had two such engine and drive combinations placed on opposite sides. Railcars 19-20 were fitted with a separate high-low ratio gearbox on the final drive side of the gearbox. This allowed a top speed of about in high and about in low. Railcar W20W retains this in preservation. Brakes The brake system on railcars No. 2, 3 and 4 was unconventional. Instead of the usual vacuum actuated tread brakes used on most British rolling stock of the period, an automotive style system was adopted, utilising vacuum-hydraulically actuated drum brakes. A vacuum brake cylinder - hydraulic master cylinder set was mounted on each bogie. From the master cylinder, hydraulic fluid passed through hoses to the operating cylinders. The operating cylinders actuated cam mechanisms within the brake drums to apply the internally expanding brake shoes to the inside circumference of the brake drums. The 20 inch internal diameter cast steel brake drums were bolted to the inside face of one wheel per axle. The vacuum necessary for brake operation was created by three rotary exhausters, two being driven directly from the engines and one by each engine. The third exhauster was chain driven by the final drive shaft. This arrangement allowed vacuum to be maintained during coasting, when the engines would be shut down. The system operated at a vacuum of 22 to 24 inches of mercury. This vacuum being stored in four reservoirs on the chassis. Another unusual aspect to this system was that vacuum was increased in the vacuum brake cylinder to apply the brakes. This being contrary to normal british railway practice, in which vacuum is reduced to apply the brakes. Operational history The prototype unit, No. 1, made its first run on 1 December 1933 between London Paddington and with a large number of press representatives. Three days later this unit entered public service between , Windsor and . Soon after this the GWR ordered the next three production units, nos.