The Assabet River Rail Trail (ARRT) is a partially-completed multi-use rail trail running through the cities and towns of Marlborough, Hudson, Stow, Maynard, and Acton, Massachusetts, United States. It is a conversion of the abandoned Marlborough Branch of the Fitchburg Railroad. The right-of-way parallels the Assabet River in the trail's midsection. At the north end it veers north to the South Acton MBTA train station while the south end veers south to Marlborough. When fully completed, the end-to-end length will be 12.5 miles (20.1 km). As of June 2020, the southwest 5.1-mile (8.2 km) portion of the trail from Marlborough to Hudson and the northeast 3.4-mile (5.5 km) portion running from the South Acton MBTA station to the Maynard–Stow border are completed. No current plans exist for pav