Marty,

Can't add much more to the information provided by Glenn, Dennis, Ted and Roger, except to confirm that in a 1947 ORER I have, the 7500 and 7700 car series were listed for interchange.

Going by the AAR descriptions and diagrams shown in the 1947 ORER, they pretty much confirm that the IL of 36'10" was the platform (deck) length, while the length of 37'5" was the dimension over the strikers for both cars. The 7500's were rated for 70000 lbs, the 7700's for 100000 lbs.

Dick