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Wednesday, March 5, 2025

Royal Scot Locomotive

Painting of LMS Royal Scot 6161 "King's Own" by Murray Secretan

In service from 1930 to 1963. King's Own Royal Regiment Museum Lancaster has a page about this loco, as does Wikipedia.

3 cylinder compound locomotive steam engine
This is only image I could find of a three cylinder engine. Sad, very sad.

In my mind, steam locomotives have two cylinders, one on either side of the front of the engine. The piston in each cylinder drives the wheels on that side through a connecting rod. Turns out that some locos have three and even four cylinders. The Royal Scot has three as shown in the drawing above. This means that the straight axle between the second pair of driver wheels has been replaced with a crankshaft. The center cylinder is slightly elevated so that the connecting rod clears the axle of the first pair of driver wheels.

This post on on the Trains Forum page about three clinder engines offers some explanation:

The three cylinders are high on my list of unique designs that never took off over here in the US.

3-cylinder locos had a speacial exaust chuff as well, from the four chuffs per driver revolution on normal engines, 3 cylinders had six chuffs, making for some very interesting “stack talk”

While here in the US they never really took off, they became fairly common across Europe. Among some notable European 3 cylinders is the A4, thats right, the “Mallard” was 3 cylindered, along with her sisters who still operate today!

[3 tiny pictures that purport to show the center cylinder of 3 cylinder locomotive. They made no sense to me. Perhaps they would if you were familiar with these locomotives.]

The center cylinder yielded more power in a smaller locomotive, the #12 (pictures above) is an 0-8-0, but still produced more than 60,000 lbs tractive effort! Thats more than some 4-8-2’s and 4-6-4’s of the time period!

Unfortuantely, the 3rd cylinder was difficult to service, and the costs related to maintenance outweight those saved by fuel economy. These engines became known as “Roundhouse Queens” as they sometimes spent longer under repair than in operation.

This is probably more than you wanted to know,

The Great Western Archive has audio recordings of a variety of steam locomotives. Simple, i.e. not compound, engines sound different than compound engines.

For a while I was thinking that steam locomotives could have been improved by using steam engines with several smaller cylinders running at high speed, like a diesel engine, but that would have meant using gears to transmit power to the wheels, and making large gears that would take the load may not been economically feasible in early 20th century. The Shay locomotives built for logging industry did in fact do this, but they weren't carrying the enormous loads that mainline freight trains did. Diesels got around this problem by using electric generators and motors. I suspect electric motor technology in the early 20th century wasn't up to the task.

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