Sorry, not RR & not 'Express'. It's short for 'EHKSPEDITSIYA' which is a Dispatch Office. In your case it's the Warsaw/1st Dispatch Office/canceller 3. (WARSZAWA/I EXP/No3).
My reference is "Russian Postmarks" by A.V. Kirvushkin & P.E. Robinson, I think I got mine from Barefoot, an invaluable general intro to Russian postmarks, which, of course, includes Poland.
From this we learn that this type of postmark was used in some of the dispatch offices of St. Peterburg & Warsaw towards the end of the 1880's. They were used only for cancelling stamps & did not include a date as they were always accompanied by a separate datestamp.
Here's a postal card with a separate Warsaw datestamp.

1903 20 Dec Warsaw to Vienna
I've found three examples Down Under, so they can't be that rare