HO Scale Accessories

28" wheels are used on modern Tri-Level Autoracks, some Bi-Level Autoracks, and some older intermodal cars such as Impack spine cars, front runners & four runners - check your prototype.
33" wheels are used on 50 & 70 ton capacity cars (CAPY approximately 150,000 LBS or less).
36" wheels are used on 100 ton capacity cars (CAPY approximately 200,000 LBS), and on all passenger cars no matter the era.
38" wheels are used on 150 ton capacity cars, primarily the center connection trucks on modern intermodal cars with 33" wheels at the coupler ends.
Cabooses can use either 33" or 36" wheels depending on the prototype.

The CAPY information is typically printed on model freight cars and can be used as a guide. Some cars have the wheel size printed on the sides or ends of the car in the data. If you are ever unsure of which size to use, you can simply replace the wheels currently on the car with the same size.

Wheel diameter can be measured with a caliper:
28" wheels measure (28/87.1=.321") in HO scale.
33" wheels measure (33/87.1=.378") in HO scale.
36" wheels measure (36/87.1=.413") in HO scale.
38" wheels measure (38/87.1=.436") in HO scale.
(87.1 = HO scale to prototype ratio. HO scale is 87.1 times smaller than the prototype).