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Compound Steam Locomotives

  • I've recently read that compound steam locomotives stopped being built beginning around 1910 after the incorporation of superheating into new locomotives. But, I see plenty of modern steam locomotives with two sets of cylinders and pistons on both sides of the loco. If these are not compound steam locomotives, how are the extra cylinders being used?

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  • Jerry Roberts
    I've recently read that compound steam locomotives stopped being built beginning around 1910 after the incorporation of superheating into new locomotives. But, I see plenty of modern steam locomotives with two sets of cylinders and pistons on both sides of the loco. If these are not compound steam locomotives, how are the extra cylinders being used?

    Compounding is the use of steam two times.  Most steam locomotives were built as 'simple' where the steam was only put to the driving cylinders one time and then exhausted through the smoke box and out of the locomotive.  

    When you see a articulated locomotive while most were built as simple, some such as the C&O 1309 - the last engine Baldwin built for a US carrier were built as compound engines.  On compound articulated locomotives the front engine is normally the 'low pressure' engine, and the rear engine is the 'high pressure' engine.  Steam gets routed to the rear engine first at the normal operating pressure of the boiler, after that steam has performed its duty in moving its piston it gets exhausted to the front engine at a lower pressure than it entered the rear engine.  Because of the lower pressure of the steam entering the front engine, the front engine's pistons are larger in diameter than the rear engine.  Once the front engine has used the steam to move its piston the steam is exhausted into the smoke box and out of the locomotive.

    The Big Boy UP 4014 is a articulated locomotive and both its engines are simple - they use the steam and exhaust it after use.  In most cases, if you look at the top of a simple articulated locomotive you will see two separate stacks -one for each engine.  On the C&O 1309 that is being restored to operation by the Western Maryland Scenic there is only a single stack.

    European's tended to use compounding on many of their non-articulated locomotives.  In most cases the low pressure cylinders are mounted within the gauge of the locomotive and they are connected to the front driver axle that is constructed as a crankshaft.  Americans found the maintenance required to keep compound non-articulated locomotives to be too high a cost to pay for the ability to use steam two times.

  • Jerry, I hope you see this, so late.  

    Compound engines, often called "Mallets" after their chief designer, were made right up to 1952, as far as I know.  The last main line steam locomotive erected and placed on the rails in the USA was a Y6b #2200.  It was called for by the Norfolk & Western Rwy, and built right there in its own shops in Roanoke, VA. Balt has provided you with a great response, but I wanted to add a little history to your 1910 factoid.

    Someone wrote a duff essay if you read it and recall it correctly.