Hello John,
True, I purchased one, brilliant machine in every way.

From memory the catalogue did not offer a particular car set.
But any era IIIa set (before they got the DB biscuit logo on the side) would do.
2 axle, 3 axle, or 4 axle would not matter.
The new Insider set 42529 would be a good match too.

I use my 37115 with Trix cars 23383, 23385 and 23387 and Märklin 43085 (a Postmuseum article).
I also got another postal car to go with it, though I cannot trace the model number.
The real one is in the photo inside the wooden box lid.

I am sure there are many others in both brands.

Kimball

As an express train there are also the Roco Württemberg express wagons: 44536/7/8/9 and 45494/5/6/7 which can all be had rather cheaply second hand

Fleischmann have the Prussian express wagons of earlier wood design: 568003, 569103, 568203, 568303, 568304

Brawa have the new Polish Pikes: 45317, 45318 and the soon to be released steel Prussians: 45421, 45422

The Trix wagons mentioned are also great.

Throw in a restaurant wagon if you like, or not, such as Roco 64892.

There are also the Roco E36 wagons, 64565/6/7/8 as can be seen here:


"18 115 (Bw Heilbronn) in front of D 8 in Stuttgart-Feuerbach. (06/22/1952)"



An ideal consist would be a combination of the above wagons, since before and after the war there were many shortages and trains rarely ran with pure consists, except maybe the E30 / E36 Eilzug wagons since there was a large pool of those available and therefore also ran in D-zug and Eilzug trains.

Funnily enough the 'matching' wagons from Marklin aren't a match at all, since they fit into era IIIb after the class reform and with the DB biscuit. The 37115 is era IIIa.
Though you could argue that it was possible in 1956/57 before the 18.1 received it's third headlight and the DB logo...
Marklin never released the type 39 schurzenwagen in epoch IIIa livery bottle green (only steel blue for F-train) which was a huge shame since Liliput didn't either.


You could also drive it as a passenger train which would be suitable with Prussian compartment cars such as Kimballs photo and also as shown here:

"The elegant Württemberg C (BR 18.1) was one of the most successful designs of its kind and was used on the mountain routes of Württemberg from 1909 to 1955. Here 18 115 hauls the passenger train P 3419 in the Danube valley near Thiergarten between Beuron and sigma rings. (08/23/1953) Photo: Carl Bellingrodt"