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!
As a matter of fact, my favorite locomotive is a 3-cylinder: Alton and Southern #12
This is the 3rd cylinder, located under the smokebox, between the other two cylinders. (Alton and Southern #12)
In the foreground is the crank on Axle #3, the rod running between the frame goes to the center cylinder. You can also make out the “crooked” axle unerneath the rod. It was made this way to make room for the motion of the rod.
You can easily see where the center cylinder is here.
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,