Pretty easy on commuter trains. Locomotive front or rear if push-pull with a cab car on the opposite end and coaches in between.
On a more serious note, CNJ had some interesting variations that included an former Blue Comet open end observation to Phillipsburg until 1975. In the late 60's early 70's CNJ has as I recall a former FEC round end observation on the rear. PC and later NJDOT had a former NYC lounge car mid train up until 1983. I don't remember if it was the same train, but the Jersey Shore Commuters Club has well maintained P70 as the first car behind the locomotives until 1983 as well.
In long distance trains, sleepers traditionally have run at the rear of the train. However, Amtrak will put the sleepers forward on some trains as has been noted above.
The best advice given so far is to research the train in question. Every railroad had their preferences and you'll also discover those unique cars that don't come in most model sets.
For example, the Broadway limited had Harbor Cove and Harbor Rest. It was a 3 bedroom sleeper with a shower section, a barber shop, an onboard secretary compartment, and a lounge. That is an interesting car.
The combined 18 car El-Capitan/Super Chief which started running off season from 1958 until 1970 had a separate train behind the last EL-Cap car that contained it's own diner, dome, and all the sleeping cars.
Most importantly it's your railroad. Have fun!
