This description is from the catalogue of Baldwin erecting card drawings as published by DeGolyer Library of Southern Methodist University in Dallas, Texas:
The Baldwin class number was a rather complicated classification system initiated in 1842 and used until some time around 1940. The class number for the Nevada-California-Oregon Railway (sic) can be described in the following manner.
10-24 D 35
- The initial number is the total number of wheels of all kinds under the locomotive.
- The second number indicates the cylinder diameter in inches, this quantity being obtained by dividing the cylinder diameter by 2 and adding 3 to the quotient. The above classification number of 24 indicates a cylinder diameter of (24 divided by 2) + 3 = 15". A fraction, e.g. 42/68 indicates a compound locomotive having two cylinders.
- The letter designation indicates the number of pairs of coupled driving wheels. "A" = special class of high speed geared locomotive with one pair of driving wheels. Also rack railroad locomotives. "B" = one pair of driving wheels. "C" = two pairs of driving wheels. ... "F" = five pairs of driving wheels. Double letters = articulated locomotives having more than one set of driving wheels.
- The last number(s) in the class number identifies a specific locomotive within the class [although the numbering is not contiguous.]
Therefore 10-12-D represents 10 wheels, 9 inch cylinders, and 3 coupled axles.
I am grateful to Martin Coombs for this information.