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, Middlesex County, Massachusetts, 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. A section from School Street to Arlington Street in Watertown has been completed. A small portion in Waltham has been converted into a park called Chemistry Station Park after the railroad station once located there. Construction of an extension to Fresh Pond Reservation in Cambridge began in the summer of 2018 and is expected to be completed in 2021.