Railroad Forums
Firemen?
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Moderator: Robert Paniagua
Fireman in diesel era could have done many things such as, operate or tend the steam generator on passenger locos, deal with mechanical issues, grab orders, etc.
You just shook the hand , that shook the hand, that shook the hand of John L. Sullivan!
... and in later years there were plenty of mechanical issues requiring somebody to go back there and check on things -- and maybe troubleshoot and fix things.
Sleep while the engineer was running, run while the engineer was sleeping. 
On some early diesels they had to open and close shutters, and engage and disengage cooling fans. This was based on the weather, and how hard the engine was working. One other duty of the fireman was to watch for motions before portable radios were given to train crews.
One point that was not mentioned was that a fireman was in an apprenticeship toward becoming an engineer. The engineer could give out many pointers about how to run the train, on a given line. Often the fireman ran under the engineer’s supervision. This is how the craft used to be taught.
All of what has been posted above along with calling signals out and just being an extra set of eyes in the locomotive cab.