Normally, there was one RPO on a train. However, there were also a few instances in which the train was split en route and continued on to two different destinations. On a few of those trains, there were two RPO's before the point at which the train was divided, and one on each section thereafter. It was very common that RPO cars did not remain in the train for the complete trip, especially on some western passenger trains which ran over 2,000 miles between initial and final terminals.
And, yes, the RPO car was locked at both ends. Railway Mail Service Clerks wore small sidearms. There was a provision for a train crew member to enter the locked RPO in case of an emergency. There was a very -- I mean very -- small door which could be opened but required that the train crew member crawl and squeeze through. That way, no robber could pose as a Trainman and get the drop on the clerks. RPO cars carried registered mail, including bank bags. If I remember correctly, both the train Conductor and the head RPO Clerk had to report any entrance of the RPO by a crew member on their respective daily report forms.
