The 1943 cyc has a photo of USNX 1006, one of the GATX 28-tube cars, and also lists the OD as 18.5 in. I think I recall reading elsewhere that the wall thickness was 3/4". I can't find confirmation of that figure, or recall the source. At least it's in the plausible range considering either weight or hoop stress. So yeah, heavy pipe, but not unusually so.
The 18.5" diameter is perhaps a bit odd, since standard NPS 18" pipe sizes have an 18.0" OD.
Steve, did your measurements show 18" or 18.5" OD? What car was it?
