Record-breaking mega-tunnel for Toronto subway extension is over halfway done
A record-breaking subway tunnel being carved deep beneath Toronto streets has crossed the halfway mark, and will soon carry trains deep into Scarborough as part of a major transit expansion project.
Under construction since 2021, the 7.8-kilometre Scarborough Subway Extension (SSE) project will eventually extend the TTC's Line 2 Bloor-Danforth from its current terminus at Kennedy north to Sheppard and McCowan.
Setting the project apart from other subway tunnel builds in Toronto transit history, the costly and controversial three-stop SSE will feature a single bidirectional tunnel, unlike the twin-tunnel construction employed on other lines on the network.
As one would expect, fitting two sets of tracks in a single tunnel requires a very wide bore, and this 10.7-metre diameter tunnel — wide enough to comfortably house a three-storey building — will in fact be the largest diameter subway tunnel in Canada once complete.
Metrolinx recently shared an update on the enormous tunnel's construction progress, including a rare glimpse inside the cavernous void and the tunnel boring machine (TBM) responsible for the excavation — dubbed "Diggy Scardust" in a comedic spin on David Bowie's Ziggy Stardust alter ego.
According to the transit agency, over half of the extension's 6.9-kilometre-long tunnel has been etched beneath Scarborough streets.
As Diggy Scardust carves further towards its goal, almost 2,000 individual precast concrete tunnel liners have been installed, trailing the TBM to secure tunnel walls.
Though it will still be a few years before Line 2 trains barrel through this massive tunnel, a train is already running in this cylindrical cavern, shuttling crews and materials to and from the excavation's front line.
Current progress on the line comes after a major construction hiccup, when the TBM was lodged for months in 2024 due to unexpected soil conditions, before finally resuming its subterranean journey.
Metrolinx
