Answer:
21 – 21
4-6-6-4-4-69 ————— 404 UP
32
The number, 4-6-6-4 is the wheel arrangement. The extra “4” on the end notes that No. 3985 was part of the fourth of five orders of Union Pacific Challengers. The 69 is the diameter, in inches, of the driving wheels.
The fraction describes the drive cylinders: both the front and rear engines have cylinders 21 inches in diameter, each with a 32-inch stroke. Finally, the “404” refers to the weight of the locomotive, in thousands of pounds.
Not all articulated steam locomotives used the same cylinder size on the front and rear engines. A Mallet (pronounced Malley) locomotive uses compound expansion cylinders, where the steam is used first in the high-pressure cylinders, then used again in the low-pressure cylinders. The low-pressure cylinders typically drive the front set of drivers and are larger in diameter to obtain the same power output.
Challengers and Big Boys do not use compound expansion, so both sets of cylinders are the smaller high-pressure cylinders. The Norfolk & Western Y class uses compound expansion and is a true Mallet. — Tyler Trahan
