Metrolinx is preparing to embark on digging the single-largest subway tunnel in Toronto's 67-year history of rapid transit: the controversial 7.8-kilometre Scarborough Subway Extension is now finally underway.

The Scarborough subway will extend the TTC's Line 2 eastward into Scarborough with three underground stations, replacing the above-ground Scarborough RT and its six stations.

While most of Toronto's transit lines were designed with twin tunnels, the Scarborough extension's eastbound and westbound tracks will both run through a single 10.7-metre diameter tunnel.

Simply put, that's a huge tunnel, about as tall and wide as a telephone pole, with ample room to run twin subway tracks. And building such a massive tunnel requires an equally enormous tunnel boring machine, or TBM.

Such a machine destined for Toronto is currently being tested by manufacturer Herrenknecht, half a world away in Schwanau, Germany, a small town along the Rhine River, a stone's throw from France.

Metrolinx shared a post in early September with photos of the enormous machine being assembled. Their latest update reveals a fully-assembled monster of a machine, emblazoned with a red maple leaf on the cutting head that will forge the line's path below the streets of Scarborough.

View of the TBM during assembly in Germany. Photo by Herrenknecht via Metrolinx.

When testing wraps up, this titan will be disassembled to begin its long crossing of the Atlantic Ocean by boat, expected to arrive in Canada early next year, before making the final leg of its journey to Toronto via truck.

Metrolinx is gearing up for the arrival of the TBM in a couple of big ways. This includes a contest to name the machine launching this fall, and the actual preparation of a launch shaft where the TBM will begin its mission.

Crews are now getting their feet dirty at the corner of Sheppard Avenue and McCowan Road, working on a gargantuan pit that the TBM will eventually be lowered into, measuring 80 metres long, 30 metres wide, and 25 metres deep.

The TBM's launch shaft being excavated at the corner of Sheppard Avenue and McCowan Road. Photo via Metrolinx.

Once the colossal machine is lowered into the ground next spring, it will begin digging the line at a rate of approximately 10 metres per day.

Launch shaft for the Eglinton Crosstown LRT. Unlike the twin tunnels used for Eglinton, the Scarborough subway's two tracks will be within a single, massive tunnel. Photo via Metrolinx.

A second shaft will eventually be dug at Midland and Eglinton, where the TBM will be removed after completing the almost eight-kilometre tunnel.