From the wye junction with the Amtrak Boston line (Shore Line) at Groton (MP 0.0), where there is a yard, the single track line, with no signals and initial speed limit 10 mph in Yard Limits, turns northeast, past Fairview (MP 1.1), where there is a yard, Yard Limits end, and the speed limit rises to 25 mph, north-northwest, north alongside the east bank of the Thames River, past Submarine Base (MP 2.1) and MP 2.5, where the speed limit rises to 40 mph, north-northeast, following the river, past the siding at Allyns Point (MP 6.1), north, following the river, past the yard at Milk House (MP 10.0), MP 11.4, where the speed limit drops to 10 mph for the traversal of Norwich, Norwich (MP 12.2), where there are a number of derelict factories alongside the line, and the line turns north-northeast, still following the river, and 8th Street (MP 13.6), where the speed limit rises to 25 mph.
The line bridges over the river to its west bank, and turns east at Tafts (MP 15.4), where the speed limit drops to 10 mph for the traversal of Tafts Tunnel (MP 15.9), after which the speed limit rises to 40 mph, with a speed limit of 35 mph for curves northeast, east, and then north between MP 17.1 and MP 19.0, after which it reverts to 40 mph past the siding at Reads (MP 20.5) and Jewett City (MP 21.7), turning northeast and bridging across the river, and then leaving the river behind, turning north-northeast past the start of Yard Limits at MP 27.0, for which the speed limit is 10 mph, the yard at Plain[field] (MP 28.1), where the Willimantic Branch trails in on the west side, controlled by stop signs, and once continued on the east side, and the end of Yard Limits at MP 28.5, where the speed limit reverts to 40 mph.
The line continues curvily north-northeast, past Central Village (MP 31.3), Wauregan, Danielson (MP 37.1), and the siding at Dayville (MP 39.4), turning north-northwest to the yard at Putnam (MP 45.0), where a line once trailed in on the west side and the depot is on the west side of the line, alongside the river again for a short distance, turning north-northeast past Klondike (MP 45.4), Mechanicsville, where a line once left on the east side, turning north-northwest past Grosvenordale, north past North Grosvenordale (MP 50.5), and just east of due north past the Connecticut-Massachusetts line, Hill (MP 54.5), where the Southbridge Running Track trails in on the west side and another branch once trailed in on the east side, the siding at Webster (MP 55.0), Oxford (MP 59.8), just west of due north past Auburn (MP 56.4), and north-northeast past the yard in Yard Limits at Sword (MP 67.7), where the speed limit drops to 10 mph, Hammond Street (MP 70.1), where there is a speed restriction to 5 mph, the junction with the Main Line from Providence, on the east side, at Jackson (MP 70.5/42.6), where the speed limit rises to 20 mph, a complex of interconnecting tracks permitting a reversal of direction, and Worcester (MP 45.1), junction with the former Conrail Boston Line and the P&W Gardner Running Track.