I understand that the Burlington was the camouflaged owner of the
Packers Car Line, formed to provide Armour & Company with more modern
reefers - PCX 4000-4399 built in 1956 by Pacific Car & Foundry.

According to the 1968 ORER, there was also PCX 4400 - 4600 of similar
size. Can anyone advise when these were built and by whom?

Rupert,
Packers Car Line had 400 cars (4000-4300) built by PC&F. A photo of PCX
4142 (available from Bob's Photo) taken 12/12/57 has NEW 8-57 stencils.
In the 1/59 and 1/62 ORERs this was the only series listed.
Regards,
Ed Hawkins