I wrote:

As my feeble memory recalls, the SP O-50-13's were
12,500 gallons capacity.

My feeble memory was right for once:

http://www.railgoat.railfan.net/photos/sp/sp062899.jpg

Note the extra height of the dome as compared to the
Athearn tank car. Note too that to do this class the
stirrup steps will have to be reworked, among other items.

Shawn Beckert