Wheel sizes are another case of look at the photos, but I'll give you a bunch of rules of the thumb. Pre WWII, use 33" ribbed back wheels on any freight equipment. Use ribbed 36" wheels on
passenger cars.
After WWII car weight and bearing loads indicate sideframe and wheel size. Any car built after WWII will have smooth wheels. Ribbed wheels were banned in 1958 for interchange. By 1974 friction
bearings where all but non-existent on any cars. So, you may have to start rewheeling by changing sideframes.
70 ton, 100 ton and 125 ton are common weights for trucks. Visually, the number of springs is the quickest spotting feature. Two springs is, probably, 70 ton. Three or more is, probably, 100 ton. 125
ton trucks are a couple inches longer, stouter and have a distinct arch in the top chord so that the bolster is low enough for a standard car height. In [HO] only Eastern Car Works makes a correct
125 ton truck. The standard truck from Athearn, MDC and most others are 100 ton. LL P2K, Intermountain, Red Caboose, Atlas and others in the prototype fidelity group have 70 ton trucks and, usually,
supply the correct weight and bearing frames for the cars, AS BUILT. Note that the railroads will have changed sideframes and wheels to eliminate outlawed features.
I'm getting to the wheels!
Common uses of 70 ton frames and 33" wheels are boxcars, regardless of length, flatcars, open 40' hoppers and gondolas. 50' open hoppers will, probably, use 36" wheels. Most tank cars and all covered
hoppers are the problem cars. Auto racks MAY use 28" wheels for clearance reasons since the cargo is not heavy.
A 40' cement hopper will use 36" wheels, but a 40' granules hopper will use 33" Most of the big covered hoppers, LL - Athearn - MDC - Intermountain, will use 36", but some of the newest (1980's) will
use 38" in 125 ton sideframes. Rule of the thumb is 33" wheels on 40' cars, 36" on bigger cars.
Modern (1960 on) tank cars are sized for the density of the cargo. A 20' "beer can" carries the same weight as a 60 footer. 36" wheels on all. Anhydrous ammonia cars will be, generally, 125 ton
trucks with 38" wheels.
Beyond these rules of thumb you can look at the lettering on the better kits and models. If it is not a 70 ton car the wheel diameter will be stenciled on the end, or above the trucks!
Christian