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A true Mallet uses compound expansion: live steam is fed to the rear set of cylinders (red), and the used steam from this set is fed to front set of cylinders (orange) for a second expansion before escaping via the exhaust (yellow).


Compounding articulated locomotives has several advantages

  • Steam is used twice thereby reducing fuel consumption (up to 40%)
  • The moving joints in the tubing feeding the front cylinders can be kept steam tight more easily due to the lower working pressure
  • The system has built-in balance against slippage. If only the front truck slips the pressure in the connecting (orange) tube will suddenly fall stopping the slipping due to the lack of steam supply. If the rear truck slips the pressure in the connecting tube will suddenly rise also stopping the slippage due to the back pressure. If both sets slip the engineer should just throttle down.

Compounding also has disadvantages

  • It demands a far more complex system of steam feeding.
  • When starting a compound locomotive both sets of cylinders need fresh steam because there is no or insufficient steam in the connecting tube during the first beats.
  • The front set of cylinders is larger in order to deliver approximately the same power as the rear set. With very large engines like the Y3 these front sets become so large that it becomes increasingly difficult to fit in the loading gauge.

Essentially two sets of cylinders in a compound system deliver less power then two sets with single expansion. So if you want power and care less about fuel consumption single expansion is a logical choice: you can feed the front cylinders with fresh steam, increasing power because of the the higher pressure. And you can increase the size of the rear cylinders to match the front set until they reach the limit of the loading gauge, also increasing power because of the larger size. So when larger and more efficient boilers became available it was a logical step to abandon the compounding system to the favor of raw power, as happened with the giants of steam like the Big Boys, the Challengers and the Allegheneys.