Where does “толкач” come from?

толкач (Russian) comes from Russian -а́ч, from Old East Slavic -ачь, from Proto-Slavic -ačь — Denominal, forming nouns denoting a carrier of a...

толкач (Russian): helper engine, pusher engine, bank engine (a railway locomotive that temporarily assists a train that requires additional power or traction to climb a gradient)