The basic rule of M&E consists is that there are no rules. Whatever kind of baggage or RPO cars a railroad used might appear in an M&E trains. BTW, all cars handling mail, express or baggage were listed in the equipment register as baggage cars.
You need to decide what your railroad used as rider cars. The options are
- caboose with high speed trucks and passenger brakes (rare except on the PRR).
- obsolete passenger car converted on rider coach (on the NYC, middle windows were covered.)
- Any passenger car not needed for revenue service.
The minimum requirement was seats for the conductor and rear brakeman and some means of heating the car.
As for cars from other railroads, it was mostly a matter of where you had through cars. Magazine distribution, for example, could be a million copies nation-wide from a single printing plant. That's a lot of carloads, maybe once a month and some destinations beyond the origin railroad receiving enough copies to fill a car. If there was enough parcel post from New York to fill cars to LA and/or SF , then there would be an NYC or PRR car on the SF, Q, CNW, MILW, UP or SP and v.v.
So just think about what high volume routes might have used your railroad and select foreign cars accordingly.
