I was talking to an Amtrak conductor one day and he said that the modern diesel-electric engines will easily pull 8 or more cars. The problem, he said, was that the generators are only designed to power 4 cars. They are usually pulling 6 cars and it taxes the generators.
When the bad old private railroads ran passenger trains, each car was equipped with a wheel generator which charged a bank of 32 volt storage batteries; the movement of the train kept the batteries charged. While the generator added drag to the axle, it didn't require that the locomotive be equipped with an auxiliary generator.