[2] At grade, it runs approximately west–southwest until it crosses over the (underground) Line 1 Yonge–University of the Toronto subway at Snider West (milepoint 1.2),[2] just south of Highway 407 station.

At the municipal border between Vaughan and Toronto, it descends into a tunnel below the intersection of Steeles Avenue with Martin Grove Road.

The line continues southeast through a residential area of Milton, crossing over Main Street West and Derry Road, then under Louis St. Laurent Avenue.

Returning to farmland, it crosses Britannia Road at grade and reaches Ash (linked to the planned CN Milton Logistics Hub) at milepoint 39.5.

It crosses Side Road 1 (also the Milton–Burlington municipal boundary) at grade, then passes over Highway 407 and Bronte Creek, reaching Tansley at milepoint 43.1.

It reaches an industrial area of Burlington, then crosses over Walkers Line and Tuck Creek, continuing to curve west.

Nearing its terminus, it runs parallel to the Queen Elizabeth Way, crossing under Guelph Line, North Service Road, and then the Queen Elizabeth Way itself as it curves south and west again, crossing under Plains Road East and over Brant Street to meet the CN Oakville Subdivision at Burlington West junction at milepoint 49.4,[2] the southern terminus of the line, which is directly southwest of the Burlington GO station.

CN began the first recognizable commuter services along the line in the 1950s between Guelph and Toronto, with one eastbound morning run and a westbound afternoon return.

[5] By the end of the 1970s, all remaining Canadian National passenger services were either discontinued or transferred to a new crown agency, Via Rail.

[8] In 1990, the International was re-routed along the North Main Line between London and Toronto, travelling along the central part of the Halton Subdivision.

In a project jointly funded by the cities of Mississauga and Brampton, CN, and Metrolinx, Torbram Road, a major four-lane collector road connecting Brampton and Malton, would have its level crossings with both the Halton and Weston subdivisions eliminated in favour of subway underpasses under both rail lines.

The section used for passenger service is an east–west stretch (part of the historic CN North Main Line) running through Brampton and Halton Hills.

As of 2021[update], passenger services along the line consist of Via Rail's daily intercity Toronto–Sarnia train (part of the Québec City–Windsor Corridor), which stops at the Brampton and Georgetown railway stations, as well as weekday regional/commuter trains operating as part of GO Transit's Kitchener line, which also stop at Bramalea and Mount Pleasant.

Metrolinx, the parent agency of GO Transit, proposed a freight bypass in the mid-2010s to create more room in the schedule for additional passenger trains along the existing section of the Halton Subdivision.

[14] By 2021, Metrolinx had decided against pursuing the freight bypass in favour of greater co-production with CN in utilizing the existing line.