I posted several replies about this trailer to the OPs Facebook post about this trailer. I will only repeat some of it here.
I believe the OP has found the 23rd of about three dozen 35' Airstreams built for Southern Pacific Railroad in 1960. I believe the serial number is 35S-1023 not 33S-1023, though I could not clearly read it on my phone screen. Airstream did not build a 33S-1023, as they only built 14 33-footer Airstreams for Southern Pacific RR in 1960. It likely has a 33' body which would be 35' including the tongue. This trailer was ordered on January 4, 1960 and likely built a few days later. The build month of January 1960 is stamped into the serial number plate as "160".
BTW, the Vintage Airstream Photo Archives are very incorrect regarding these trailers. Much speculation and little fact. The production numbers listed are not correct.
Wally Byam Caravan Club International Historian
Vintage Airstream Club Historian