The best I can do with the maps at hand is: Leaving the G&U at Williams St. following Williams to Main St. then, Main St. into town. Coming out is less clear, maybe Grove St. but possibly Milford to Brooks or. possibly PRW.
there was a topo map in 1900 reprinted without update in 1914 but, that was too early and, after that there are only very small scale maps in trolley or highway guides. The loop was operated by the Milford & Uxbridge St. Ry. which operated the passenger service on the G&U. The loop unlike the rest of the G&U was a pure trolley line without RR freight.
In 1900 there was a spur from where the G&U crosses Grove St. to Main St. along the approcimate line of Station Rd. but, I dont think it was used for the trolley line which is shown, fairly clearly, going up Main, to at least, Grove St. There is less historic record of trolleys in that part of Massachusetts than any other.