Tower A to Willows

From the interlocking at Tower A (MP 0.3), on the north bank of the river, where there are crossovers and the lines split three ways: the two-track Fitchburg Route Main Line (Fitchburg Line) heading west-northwest; the two-track New Hampshire Route Main Line (Lowell Line), continuing northwest; and the three-track combined Eastern/Western Route Main Lines (Newburyport/Rockport Line and Haverhill Line) heading north-northwest, the Fitchburg Route Main Line, double track, current-of-traffic (Rule 251), maximum speed 60 mph for passenger trains and 40 mph for freights, but local restriction 10 mph to MP 0.8 and 20/10 thereafter, passes a grade crossing at Engine House (MP 0.7), crossovers at Tower H (MP 1.3), where Two Main Tracks, CTC, begin, and the speed restriction becomes 40/25, a bridge carrying a former B&A freight line overhead, Union Square, a grade crossing at Park Street (MP 2.7), Somerville, a spur on Track 1 at Spagna (MP 2.9), and the passenger station at Porter Street, Cambridge (MP 3.4), where there is a 40/25 speed restriction on the curve through the station.

The speed restriction becomes 35/25 at Walden Street Bridge (MP 3.71), past a grade crossing at Sharman Street (MP 4.2), crossovers at West Cambridge (MP 4.2), where Track 2 becomes current-of-traffic (Rule 251) eastbound, a branch curves away on the south side the the Western Electric Plant, an erstwhile line once turned away on the north side, headed for Bedford, and the West Cambridge yard begins on the south side, Alewife Brook Parkway (MP 4.59, where full line speeds prevail, except a 55 mph for passenger trains on Track 2 to Hill, crossovers (25 mph on the crossovers) at Hill Crossing (MP 5.5), where the yard ends, an erstwhile line once trailed in on the north side from Somerville Junction, and both tracks become current-of-traffic (Rule 251) and full line speeds prevail.

The line turns curvily southwest, past a grade crossing at Brighton Street (MP 5.7), with a 55 mph passenger speed restriction on Track 1, the passenger station at Belmont (MP 6.4), a 45 mph passenger speed restriction on the curve from MP 6.4 to MP 6.7, the passenger station at Waverley (MP 7.4), a grade crossing at Beaver Street (MP 8.3), where there is a 50 mph passenger speed restriction on Track 1, crossovers at Clematis Brook (MP 8.3), where a branch leaves on the north side for North Waltham and a 50/40 speed restriction on Track 1 starts (to MP 9.45), turning west where a branch from Quincy Market trails in on the south side, past a 30/25 speed restriction, from MP 9.45 to west limits, Waltham, a grade crossing at Elm Street (MP 9.8), a grade crossing at Moody Street (MP 9.9), and crossovers and passenger station at Waltham (MP 9.9).

There is a turn curvily northwest, with a 40 mph passenger speed restriction on the curve from MP 11.2 to South Street grade crossing (MP 11.5) and a 50 mph passenger speed restriction, on the curve from South Street grade crossing (MP 11.5) to MP 12.0, past the passenger station at Roberts (MP 11.5), a former crossing with an erstwhile line west to Clifton Junction, a grade crossing at Church Street (MP 13.2), the passenger station at Kendal Green (MP 13.2), a grade crossing at Vile Street (MP 13.7), the passenger station at Hastings (MP 13.7), a grade crossing at Conant Road (MP 14.3), the passenger station at Silver Hill (MP 14.7), a 55 mph passenger speed restriction, on the curve from MP 15.5 to MP 16, a grade crossing at Tower Road (MP 15.6), a grade crossing at Wayland Road (MP 15.9), a grade crossing at Great Road (MP 16.0), a grade crossing at Lincoln Road (MP 16.7), and the passenger station at Lincoln (MP 16.7).

The line turns west, with a 40 mph passenger speed restriction, on the curve from MP 19.8 to MP 20.3, past a grade crossing at Sudbury Road (MP 20.0), a grade crossing at Belknap Street (MP 20.1), the passenger station at Concord (MP 20.1), Concord Crossovers (MP 20.4), a 45 mph passenger speed restriction on Track 1, from the grade crossing at Baker Avenue (MP 21.6) to the grade crossing at Conant Street (MP 22.5), a shortline heading north to Acton where an erstwhile New Haven branch (of which the shotrline operates a part) once crossed), the passenger station at West Concord (MP 21.9), the grade crossing at Commonwealth Avenue (MP 22.0), a 45 mph passenger speed restriction on Track 2, from MP 22 to the grade crossing at Conant Street (MP 22.5), the grade crossing at Parker Street (MP 24.1) , and the crossovers and passenger station at South Acton (MP 25.3), where single track, CTC, starts (25 mph on the switch), maximum speed 40/25, an erstwhile line to Marlborough Junction once turned away south, and the main line turns north.

There is a grade crossing and signals at Martin Street (MP 25.6), a grade crossing at Central Street (MP 26.0), a grade crossing at Massachusetts Avenue (MP 26.8), a grade crossing at Arlington Street (MP 26.9), a grade crossing at Central Street (MP 27.3), a grade crossing at Sargent Street (MP 28.2), a grade crossing at Depot Road (MP 28.9), a grade crossing at Foster Street (MP 30.1), a passenger station at Route 495 (MP 30.1), a grade crossing at King Street (MP 31.5), Littleton (MP 31.5), where the line starts a leisurely curve around to the west, a grade crossing at Willow Road (MP 33.5), and the junction at CPF-WL (MP 33.72, with the Pan Am Freight Main Line coming in on the north side.