The Watertown Branch Railroad was a branch loop of the Fitchburg Railroad that was meant to serve the town of Watertown and the City of Waltham, as an independent short line railroad; it also serviced the Watertown Arsenal. The line has been formally abandoned and portions are being converted into a rail trail, the Watertown-Cambridge Greenway.
Phase I was completed in 2011, connecting School Street to Arlington Street, in Watertown, with a branch to the Arsenal Mall. Phase II construction started in October 2018 and now connects with Phase I at Arlington Street and continue just past the Cambridge Water Department Facility.