A very quick trip through some of the prewar car sizes:
The first generation 800 series frame is 5 3/4" long, the second generation frame measures 6 3/4" or 6 1/2" (1930 on wards when the round ends were squared off) long and are also 1/8" wider.
Note: There is the 820 series (not to be confused with an 800 series) that measures 7 1/4" and has 4 wheel trucks.
Note: There is a 650 series that use 6 1/2" frames with four wheel trucks.
The 2650 series uses similar 6 1/2" frames to the 650. The prepended 2 to the number generally loosely signifies a 650 series car with automatic couplers upgrades and additional body styles.
Then enters the 810 large series of cars with 8 13/16" long frames.
Add automatic couplers and more body designs and you get the 2800 series of large freight cars.
