The total empty car or tare weight for weighing purposes including the wheel assemblies. But the relationship may not be true. There are many cases among specialized and flat cars that are exceptions. Note the car appears to be over weight by 200 lbs. The journal on friction bearing trucks is the axle surface that turns inside the bushing in the side frame.
Axle loading is determined from the journal diameter and length.
With the exception of wheel treads, the remaining fabrication of the trucks is free-lance following AAR specifications. The table following shows standard AAR classes with common truck arrangements and the total weight permissible in each case. It covers almost all friction bearing freight, most passenger and some tender trucks. Roller bearing journals apparently follow the same classes. The table below contains the seven standard ARR classes along with various in use truck combinations found in car builders dictionaries, cyclopedias and other sources.