Some Transition Era Operations Questions...

Another thought that occurred, a non-sequitur really, is that I wonder if any oddball shorelines tried for handing mail in a caboose? I know mail contracts were often what kept mixed\passenger trains on lines, and I know some lines did actually have "mixed" trains where the one or two intrepid travelers who braved them rode in the caboose, so it kind of surprises me that no one tacked a mail catcher on a caboose and called it a day, that I know of? Making the often state required caboose a revenue, vs non-revenue car, seems like the sort of thing RR accountants would love.  

In the real world, it would be pretty unlikely you'd have an entire branch line with all the spur tracks facing the same way. More likely, you'd have some of both. If possible, the crew would switch the trailing points going up the branch, then turn back and switch the facing-point turnouts - which of course, were now trailing point spurs - when going back down the branch.

I grew up on a branch line, there was a turntable at the end of the line in steam days; after they dieselized they just used a run-around track. The engine would be in the lead both ways, with the caboose behind.

The US Post Office had strict requirements for RPO cars - had to be made of steel, had to be completely isolated from the rest of the train - or even isolated from other parts of the car it was in, etc. I can't see any way a caboose could have been made to come up to specs to be allowed to be an RPO. I know some railroads had cabooses with a side door so it could load and unload less-than-carload stuff. I suppose maybe on a really remote line the caboose might carry sacked mail along with express??

Lots of different things here, so far.

When there are two main tracks, each one specified in the Timetable as having a current of traffic in one direction (i.e., Eastward Main Track and Westward Main Track) there have to be crossovers for use when it is occasionally necessary to run a train against the current of traffic.  Those crossovers between Main Tracks are normally trailing point for both tracks.  So, an eastward train backs through the crossover at location 1, then runs eastward on the Westward Main Track, against the current of traffic to location 2, where it returns to the Eastward Main Track, using a facing point crossover.  The reason for orienting all of the crossovers between Main Tracks in that fashion is to minimize exposure to facing point turnouts (switches) when trains are operating normally, on the Main Track specified for use in their assigned direction of travel.

R.R.B. Mail (railroad business mail, not requiring a stamp and only handled between stations of the railroad company) was handled and sorted by the Train Baggageman.  Postal employees never touched it.  There were envelopes specially printed for that use, but a plain envelope could be used by printing the letters "R.R.B." in the upper right corner where a stamp would have been affixed if it had been U S Mail.

As wjstx pointed out, there were requirements for mail cars.  Also, railroad employees were not allowed to handle    U S Mail.  Those who handled U S Mail were employed by the Railway Mail Service, a part of the Post Office.  Even storage mail cars (baggage cars in which sacks of mail were transported after having been sorted at the Post Office) were sealed, and it was Post Office employees who loaded and unloaded them.

Some cabooses were combination cars and handled Railway Express Agency shipments, making those cabooses revenue cars.