There was no variation in this scale, you see all the coachbuilders had developed tools at that gauge because they had to build coaches that would run on the terrible European roads. In a never ending circle all the ruts were 4 feet eight and a half inches apart, because all coaches were built on that scale. Carriages were made to that specication because if they didn't fit the ruts in the road they might become damaged. All roman chariots were built, you guessed it, at 4 feet eight and half inches wide. So four feet eight and half inches is basically two horse ass lengths.
Every steam engine needs a boiler to make steam from water.
In order to generate steam fast enough to supply the engine, most boilers have tubes, either fire tubes or water tubes. These provide a large area of heating surface. Early boilers and engines had to change about 30 pounds of water to steam every hour to supply one horsepower. Modern types do better, altough efficiency varies widely according to type. Fuel consumption can be reduced by preheating water, and air for the fire, with heat from exhaust steam.