John,
In most cases on most roads, any baggage car ahead of the RPO would be a mail storage car. Such cars may or may not be labeled as such.
And yes, mail storage cars could be on either side or both sides of the RPO.
Passenger baggage, and REA cars would be after all of the post office controlled cars.
Yes, a train run primarily for mail would likely have mail storage cars on both side of the RPO and/or multiple RPO's.
REA cars can and were sometimes ahead of RPO equipment depending on switching requirements at major terminals on that road. But since RPO equipment was often moved quickly to a post office siding, they were typically first.
Sheldon
That is an interesting collection of passenger cars. My main interest was for my secondary passenger trains which will carry an RPO and a single baggage/express car alone with a coach or two and maybe a diner. I run one train that is mainly mail and express with a single coach at the end. I place the RPO on that train in the middle of the storage mail cars. I see one train lists a diner but no passenger carrying cars so I am guessing the diner is deadheading back to its point of origin. I also find it interesting that some list only a parlor car as passenger carrying equipment. I know what these cars are but have never been sure how they were used. I had always guessed they were used an the first class passenger trains as a place for passengers to congregate but not as their primary accomodation. From the consists youu list, it appears some were used in lieu of coaches.
You are correct in presuming there was no hard, fast rule on RPO placement. Most railroads made up their passenger trains to permit the simplest switching moves, en route or, at the final terminal. Most RPOs would be shifted to a postal facility while baggage would be worked at the station, so it made perfect sense not to bury the RPO behind the baggage while the baggage was being worked. Passengers did not want to wait for their baggage while the switcher set out the RPO, and then brought the baggage car back to the depot. Even if the "baggage" car in fact, was sealed and carrying storage mail, it would be against the passenger carrying cars on most trains.
I looked at the New Haven's 1962 consist book, specifically the trains that had RPOs, which differentiates between baggage and storage mail cars and found the following: note, entire consists are not listed, the RPO is the car of interest.
Train 2- RPO, mail storage, baggage, coaches.
Train 3-RPO, baggage, coaches
Train 8-Sealed storage mail, RPO, diner.
Train 29-Deadhead baggage, sealed storage mail, RPO, parlor car.
Train 30-Baggage, RPO, baggage
Train 32-Storage mail, RPO, storage mail.
Train 58-Express, RPO, express.
Train 69-Express, RPO, baggage.
Train 60-Deadhead grill car, RPO, storage mail, baggage.
Train 66-Baggage, RPO, parlor.
Train 71-RPO, baggage.
Train 93-Storage mail, RPO, baggage.
Train 168-Baggage, RPO, combine.
Train 169-Storage mail, RPO, combine.
Train 171-RPO, baggage.
Train 174-Express, RPO, parlor.
Train 176-Baggage, 2 RPOs, storage mail.
Train 179-Storage mail, RPO, baggage.
Train 180 (M&E)-Storage mail, RPO, baggage
Train 181-Baggage, RPO, baggage.
Train 186-Express, RPO, baggage.
Train 187-Express, RPO, storage mail, baggage.
Train 191-Storage mail, RPO, baggage.
Train 197-Storage mail, RPO, baggage.
Train 199-Storage mail, RPO, baggage.
Note the RPOs coupled to all variety of passenger carrying cars. Note also the movement of deadheads. Sometimes they were behind all of the passenger carrying cars. Deadheads included all varieties of passenger train cars, with some trains having a many as six deadheads, ahead of the revenue headend cars. It all depended on where they were coming from and going to.
Many railroads had more traffic in one direction, than the other. Although all of those cars would not be necessary going the other way, how would you get them back "home"? Deadhead them! Just place them so they don't interfere with any en route switching.
[Deleted - Thesis not cogent]
Returning to model railroading after 40 years and taking unconscionable liberties with the SP&S, Northern Pacific and Great Northern roads in the '40s and '50s.
On passenger trains that carried an RPO and a single express/baggage car, which would normally be right behind the locomotives or did each railroad have their own rule on this. I've scanned dozens of passenger trains from the 1950s and there doesn't seem to be a hard, fast rule. It looks like the more common arrangement would be to have the baggage/express first followed by the RPO but I have looked at such a small sample size it's not clear to me that is true.