The Assabet River Rail Trail (ARRT) is a partially-completed multi-use path running through the cities and towns of Marlborough, Hudson, Stow, Maynard, and Acton, Massachusetts, United States. As a conversion of the abandoned Marlborough Branch of the Fitchburg Railroad, it is a rail trail. The right-of-way parallels the Assabet River in the 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 August 2018, the southwest end, 5.1 miles (8.2 km) of the trail from Marlborough to Hudson is completed, and the northeast end, 3.4 miles (5.5 km) running from South Acton train station to the Maynard/Stow border is completed. No current plans exist for paving the middle 4.0 miles (6.4 km) in Stow and Husdon.