The USN 61-xxxxx etc numbering are for cars NOT for interchange service . The USNX xxxxxx cars are FOR interchange service . Many of the USNX cars later in their life , which may be a short time , are reclassified USN and wind up captive to a base and get renumbered xx-xxxxx . I have been building and researching , for the last week , a dozen door and a half 50 foot cars . The info is hard to find and hard to prove via photos . I am thinking silver cars for the 50's era . Finding USNX rolling stock other than helium care so far has only come up with one from 1946 . In the early 50's Hastings Nebraska had a NOD Naval Ordinance Depot fed by UP tracks . THAT is my focus now .