A limited number of prints from the Railroad Bridges 2017 set are available as singles.
#1 - Norfolk & Southern RR Bridge and Chicago & Western RR Bridge
#2 - St. Charles Air Line Bridge
#3 - Calumet River Bridge #6
#4 - C&NW Railroad Deering Bridge
Numbers here correspond to the image #s, except for 5 which is a detail shot of #1.
Note, in case you care about such things, that the singles are drawn from the back end of the editions (higher numbers) and the sets from the front (Low numbers.) I can't promise matched #'s from incomplete sets.