Ottawa may have its own complicated transit history with the O-Train, but a project unfolding beneath Toronto streets is rewriting the record books for Canadian subway construction, and it's worth paying attention to.
A Tunnel Built for the Record Books
The Scarborough Subway Extension (SSE) has officially crossed the halfway mark, with over 3.5 kilometres of a 6.9-kilometre tunnel now carved beneath the streets of Scarborough. When complete, the extension will stretch the TTC's Line 2 Bloor-Danforth north from Kennedy station to a new terminus at Sheppard and McCowan, a three-stop addition that's been decades in the making.
What makes this project truly remarkable is its sheer scale. Unlike every other subway line in Toronto's history, the SSE uses a single bidirectional tunnel rather than two parallel bores. That means both sets of tracks, trains heading in opposite directions, share one massive underground corridor.
Meet Diggy Scardust
The tunnel boring machine behind all this digging has been affectionately named "Diggy Scardust", a playful nod to David Bowie's iconic Ziggy Stardust persona. And this machine earns every bit of its star billing.
At 10.7 metres in diameter, Diggy Scardust is carving a hole wide enough to comfortably fit a three-storey building inside. Once complete, this will officially be the largest diameter subway tunnel in Canadian history. Transit agency Metrolinx recently shared a rare look inside the excavation, revealing a cavernous space that's equal parts engineering marvel and underground spectacle.
Construction began in 2021, and the project has moved steadily forward despite being one of the most costly and controversial transit investments in Toronto's history.
Why Ottawa Should Care
For Ottawa residents who've lived through the highs and lows of the O-Train expansion, the delays, the derailments, the eventual triumphs, the SSE is a reminder that big transit projects are never simple. Canada's two largest cities are both in the middle of generational infrastructure bets, betting billions that getting more people onto trains is worth the disruption.
Ottawa's own Stage 2 LRT expansion has added new lines reaching Barrhaven, Stittsville, and Orléans, transforming how tens of thousands of residents move around the city. The Toronto tunnel, with its record-breaking diameter and single-bore design, represents a different kind of engineering challenge, but the underlying ambition is the same: make transit faster, more reliable, and more capable of handling a growing city.
As Diggy Scardust continues grinding forward beneath Scarborough, it's a good reminder that Canada is quietly building some impressive underground infrastructure, even if it takes longer and costs more than anyone originally planned.
What's Next
Metrolinx hasn't announced a firm completion date for the tunnel boring phase, but with more than half the route complete, the SSE is firmly past its midpoint milestone. Three new stations, at Lawrence, Scarborough, and Sheppard/McCowan, will eventually serve the line when the extension opens.
For now, Diggy Scardust keeps digging.
Source: blogTO via the Scarborough Subway Extension Instagram account (@scarbsubwayext).
