Mixed Passenger & freight trains. How were they sorted? - Model Railroader Magazine

Where the passenger car was in the mixed train depended on the time of year and how the car was heated.

Modern passenger cars, as used by Amtrak, use Head End Power (HEP). The engines generate electricity that is sent back to the cars for heating, air conditioning, lights etc. But for much of the 20th century, the cars were heated by steam heat from a steam locomotive, or a diesel with a steam generator in it. Lighting was from batteries under the car. In the 19th century, cars generally had their own coal-burning stoves for heat, and burned oil lamps for light.

If a mixed-train used a car needing steam for heat, and the weather was cold, the passenger car would be put directly behind the engine so it could be connected up to the steam line. Freight cars would not have steam line connections to allow the steam go back to a passenger car on the end. In that situation, there would usually be a caboose at the end of the train, behind the freight cars.

If however the mixed train used an older car with a stove, it could run it at the end of the train since the car could heat itself. That's part of the reason why even into the 1940's you could find old wood passenger cars from the 1800's still in service, since they could used at the end of a mixed train (thereby not requiring a caboose.)