...an essential resource:

google:

"United States Official Postal Guide"

or

"Official Register of the United States"

...and then, select "Books" at the top toolbar.

Here's a page from the Official Register of July, 1881:

This is an important start to figuring out whether the Pomeroy & Newark RR was earning POD revenue for its parent, the Pennsylvania RR.

Only, I need to determine what the Mail Contractors routes were.
There are dozens of routes associated with the Pennsy in this table.

Can't figure out how to find descriptions of the route numbers.
Help?!

(All the Star Routes are shown in another table, as well.)

There's another table in this publication for Railway Mail Service showing the names of the individual clerks, and which route they were assigned to. I found one entry for Wilmington (DE) to Landenbergh (PA), which is close - the P&N paralleled this line through Landenberg. Nothing for Pomeroy to Newark in that table. Just means that there wasn't an RPO for that line, I guess.