Was there a formula or dimensions or some other basis that was used
to define what constituted a "fishbelly" underframe. I ask because
while the u/f of the USRA's 40-ton DS car was obviously this type of
underframe and is usually identified as such, other car designs, the
PRR's X23/R7 for example, had a centersill that was deeper at their
center portions but narrowed towards the bolsters. Sort a "trimmer"
fishbelly if you will. I have never noticed these cars as being
referred to as having a fishbelly u/f. An even trimmer example would
be the Bettendorf u/f profile.

I am mainly trying to understand how to more precisely apply the
term, or better, when not to apply the term "fishbelly?"
Bill Welch
2225 Nursery Road; #20-104
Clearwater, FL 33764-7622
727.470.9930
fgexbill@tampabay.rr.com