From a practical point of view (i.e. not because of regulation) a five-man crew with a head brakeman would be needed on a long freight, especially if it had to pick up or set out cars along the way. A shorter train, say on a branchline or a shortline railroad, could get by with four: engineer and fireman in the cab, conductor and brakeman in the caboose. (Of course, passenger trains had a conductor and brakeman too.)
One thing that makes it difficult to discover the correct way things were done 'way back when' is that so much rail regulation was actually done by the states, not the federal government. Back before states had a DOT (Dept of Transportation) they had an agency that specifically regulated railroad rates, work rules, abandonments, adding or removing passenger trains from the schedule, etc. (Here in Minnesota it was the "Railroad and Warehouse Commission", and if I remember correctly, it's members were elected, not appointed.) This means there could be great variation in how a multi-state railroad operated in any particular place.
