The term 'Subway Power Source' can be summarized by two crossword solutions: TRAIL and RAIL. TRAIL is a 5-letter word meaning a path or track made by the repeated footsteps of people or animals, while RAIL is a 4-letter word meaning a bar of metal extending between two supports and used as a track for a vehicle or as a structural support. Both words are commonly used to describe the power source of a subway system, which is typically a network of tracks that allow trains to travel from one station to another.