Thank you so much everyone, all your comments have been extremely useful and educational.
Perhaps I should have provided some context - the terms all relate to the Italian railway network, and may sound a little old-fashioned since part of the document dates from the 1960s.
Regarding the 'winch unit' - I'm definitely looking at a photograph of a cabinet mounted on a gantry support which contains (according to the description) a handle which is turned to open and close switches on the overhead line, so that power can be turned on and off. I was hoping there would be a nice, neat term for this mechanism, but perhaps it doesn't exist in English.
With the "heating unit" term, this is being used to describe units like the Trenitalia ALn 669 or 668. There's a picture of the 668 here
http://www.railpictures.net/viewphoto.php?id=328852. The term in Italian is "automotrice termica" - I'm now wondering if this actually means an "internal combustion engine railcar", which presumably has a diesel engine which generates electricity to power the electric motors.
I'm guessing that some of this will be specific to Italy, but if anyone can help with the "winch unit" and "heat-engine/internal combustion engine railcar" terms, that would be great!