Took a quick check in my postal history sources but nothing yet. Of course if was a common RR abreviation in use it wouldn't be.
A quick check of my LIRR postal history turned up this. These seemed to be commonly used for requesting payment for shipments. As you can see it wasn't filled in on this cover but perhaps an enployee "borrowed" one for his personal use.
The placement on your cover looks like applying the stamp would have covered it up, a further mystery.
RR postal history can be fascinating. Pieces of history that you know actually were on that train.
If you start to get serious check out the Mobile Post Office Society, covers the history of mail carried on RR, trolley, buses and boats. Some very complete and detailed publications about the postmarks.
