Thanks to Al Brown, a note saved at the time is as follows:
Fri Nov 5, 2010 3:11 am (PDT)
Burlington had two classes of 50' steel auto cars: classes XA-15A (48000-48224 and 49500-49674) and XA-15B (48500-48599). There were re-numberings later. Both series of XA-15A's came with AAR spring plankless trucks (of slightly different types); the XA-15B's came with National Type B trucks. Both classes had end doors; XA-15A's had Evans auto loaders, XA-15B's didn't. This info is from the article in RP CYC 13 on Burlington's steel auto cars.
Al Brown++++++ ++++++