Model Railroading > Walther P2K Tank Cars ??

I'm unfamiliar with these cars and their differences, so would like some explanation.  The two versions are marked on the box as follows:
8,000 Gallonn Type 21, Riveted Tank Car  and  ​10,000 Gallonn Type 21, Riveted Tank Car
I'm assuming the 8,000 gal. car is shorter.  I've also seen some modelers refer to a 35' vs. a 40' tank car.  (1) Do these lengths correlate to each of the gallons; 
(2) Also, is the diameter of the tank the same between the 8,000 gal. and the 10,000 gal.?
Any help is appreciated.
Carl

The cars are the same length, but the tanks are smaller in diameter.

Posted from iPhone

That's good to know, but I wonder where the 35' vs. 40' length differences are coming from?