Hi:

In the last few days I've gotten some back issues of the RPC and taken a good look at some of the files here and on the Steam Freight Cars site, all of which has left me a little confused about how various AAR boxcars are defined.

As best as I can tell, there were four main types: 1937, 1937 (modified), 1944, and postwar. What distinguishes theses from each other? I'm pretty sure that 40-6 x 9-2 x 10-0 cars are the 1937 types and that 40-6 x 9-4 x 10-6 cars are postwar, but what are the others and how much leeway in dimensions is there within a type? For example, looking at RPC 4 pages 12 to 34 I see:

NC &STL 18850 9-2 x 10-0 "1937 AAR"
CB&Q 35000 9-4 x 10-6 "AAR" (blt 5-47)
ATSF 143510 9-2 x 10-4 "1937 (modified) AAR"
CNW 71028 9-2 x 10-5 "1937 (modified) AAR"
CNW 80606 9-2 x 10-6 "1937 (modified) AAR"
UP 197899 9-2 x 10-6 "1944 AAR"
SOUTHERN 23399 9-2 x 10-6 "AAR" (blt 8-47)
MP 37447 9-2 x 10-6 "AAR" (blt 8-57)

Thanks in advance,
KL