There were several cars, basically modified baggage cars, used by several railroads. They ran in regularly scheduled passenger trains. For example MILW 1046 (if I remember correctly) was a 70ft heavyweight express car that ran weekly between Oak Ridge TN to Hanford WA with radioactive material. Normally ran in "The Columbian", Tr. 17/18 from Chicago and cut out to go down the branch line to Hanford reservation.
In 1961 L&N had four surplus 10-3 sleeping cars modified to baggage dorms or use by Atomic Energy Commission which were renumbered 1650-1653. Only indication of use was "Return to Oak Ridge in small letters.
And there was also SCL 666 which ran Savannah to Rocky Flats CO.
