I can think of dozens of Railway Post Office cars being used as displays and exhibits. Many are done pretty well and help tell the story.

Are there any former RPO cars being that have been adapted into tourist train service and/or maintain some of their historic interiors, mail cubbies, etc?

Illinois Railway Museum and Minnesota Transportation Museum (at its Osceola, Wis., outpost) both have mail bag demonstrations. The MTM car is reasonably complete inside and has an interpreter that explains the role of the railway mail service.

Plenty in the U.K. ..........

Oil Creek & Titusville operates a former C&O RPO as a snack car. The car is reportedly still in association with the USPS.

The park train at the Portland Oregon museum used to do a RPO service at one point. No idea if it still does.

Midland Railway in Baldwin City, KS has a KCS mail car. Up until 15 or so years ago, they would put it on a train during railfan weekends. There was a group of former mail clerks from the Kansas City area that would come out and give talks to the passengers during the trips. There is a sister car on display on the turntable at the old Kansas City Terminal roundhouse. The roundhouse has been redeveloped into offices.

Tom Hamilton

The RPO at the Osceola & St Croix Valley was certainly in the consist of the regular passenger service when I travelled on the line 10 years ago as I took this photo during the journey...

Not the case for operating demonstrations at the moment as the main location - The Great Central - hasn't had the set based there in operation since October 2018, from memory.
It wasn't used during 2019 as far as I'm aware (though I didn't actually go to the Jan 2019 Gala) and hasn't reappeared since the return to "normality" post-Covid. At the moment it cannot be used for demonstration runs as there are a couple of severe speed restrictions in the area that they take place, so it can't reach the required operating speed to pick-up or drop mail (normally 45 mph). The two bridges responsible for the restrictions are scheduled for replacement in March next year, so hopefully the mail set will be able to return after that.
The Mail set has been kept away from any public access at Swithland since its last use, though the owning group have had their 1930's LNER example on display at Quorn on a couple of occasions this year for visitors to tour.....

For anyone who hasn't seen the inside of a UK example, there are a lot of similarities with the US version.....

Of the other two UK locations which demonstrate mail drops/pick ups, I believe the Nene Valley Railway have done some recently, though I'm not too sure on the current status of the set used at the Didcot Railway Centre.

CB&Q 1923, the RPO used for the demonstrations at IRM, is complete inside.

Thanks for the replies. The Oil Creek & Titusville use is really cool, and these cars do seem optimal for concession/gift car service.

I've yet to stumble over any with traditional passenger seats like one may see in a Combine. With limited windows and unusual door placement, it may not be optimal but not be impossible.

Nevada Northern has an original RPO, still set up as original.

Cumbres & Toltec has an RPO in its Historic Consist, modified for ADA service and as a concession car.

If you can, have the doors open while operating, with metal bars over the bottom portion, like a fence railing. It's nice to have a big open car that could be used for any special event, or as Earl mentions, setup like a concession/gift shop car.

I believe that East Broad Top's M1 (Brill Gas Electric car) has an intact RPO section.

When I was there for the OC&T's last Steam Weekend in 2018, I was able to get a post card, filled it out, stamped on the train and put it into the mailbox onboard the RPO car, to be mailed home. So, yes, I believe it is still associated with the USPS.

The Durbin & Greenbrier Valley's "Durbin Rocket" runs with a former D&H RPO which came by way of the Knox & Kane Railroad. As I recall the car had some tables with bench-type seating.

Alan Maples