Complex sentence: In winter, third rails were difficult to keep clear of ice, although when conditions were severe the trolley wire could also ice, and it would have to be mechanically scraped off a third rail or off a trolley wire.
Split into simpler sentences: In winter, third rails were difficult to keep clear of ice, although when conditions were severe, the trolley wire could also ice. Ice would have to be mechanically scraped off either a third rail or trolley wire.