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Here in the UK the majority of Locos designed by Sir Nigel Gresley for the LNER wre three cylinder. an earlier reply was correct in saying that the third cylinder was between the frames. The valve gear used was normally Walschearts on all three cylinders with separate geaers for each. however the grsley locos used a system that operated form the two outside cylinders via a 2 to 1 combination lever situated in front ob the cylinders. The calsses inclusde the A4 A3 for main line expresses. However due to expasion the middle cylinder sometimes 'overran' an caused damaged to the middle big end. Two classes the D49 and the B17 had the lever at the rear of the cylinders and thus negated to expansion problem. i sam building a 5in gauge B17 and watching the middle cylinder operate is a joy to behold. The cranks ( if the cylinders were parallel to each other) and had divided drive, ie the outside drove the middle axles and the inside drove the leading axle,were set at 120 degrees thus giving 6 beats per revolution. On the A4 and A3 locos the middle cylinder was inclined as all three sylinders drove the middle axle and therefore the middle connecting rod had to miss the leading axle. In this instance the spacing was slightly different in being 120 114 126. The Peppercorn classes A1 and A2 had diviided drive with three sets of WEalschearts gear. The sothern had 3 cyl locos as did the LMS 'Royal Scot 'class Any other questions email me off line. hope this helps Ron
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