Toronto's transit network is undergoing the largest expansion in a generation, with three new TTC lines and the extension of an existing line, all in varying stages of construction.

The 7.8-kilometre Scarborough Subway Extension (SSE) is forging ahead with the boring of its single 10.7-metre diameter tunnel — wide enough to house a three-storey building — which will be the largest subway tunnel in Canada once complete, carrying both eastbound and westbound trains.

While this line is a long way from complete, the massive tunnel that will add three new stops to the Line 2 Bloor Danforth is well under construction — far enough along that there is actually already a train running.

However, it isn't one of the trains that will eventually carry passengers on the SSE when it opens by 2030.

Metrolinx is showing off just the latest Toronto train that nobody is allowed to ride, joining the overdue Eglinton Crosstown and Finch West LRT lines that are expected to finally welcome riders later this year after numerous delays.

The transit agency shared a video of a small work train being used in the construction of the extension, a miniature prelude to the bi-directional tracks and much larger trains that will zip through this colossal tunnel in the years to come.

Starting at the launch site at Sheppard and McCowan, construction trains help bring tools and materials to and from the job site deeper and deeper inside the Scarborough Subway Extension tunnel.#ScarbTO #TorontoTransit pic.twitter.com/QNBPS4EnES

— Scarborough Subway Extension (@ScarbSubwayEXT) February 29, 2024

Though nowhere near the speed or capacity of a subway train, a narrow-gauge tunnel locomotive produced by Schöma, a German manufacturer specializing in small locomotives used for construction, is proving the first train to travel the still-incomplete tunnel.

This construction train shuttles tools and materials from the extension's launch site at Sheppard and McCowan deeper into the tunnel as it snakes its way toward the line's current terminus at Kennedy Station.

Once the line is complete and running full-sized trains, Metrolinx predicts the extension will see 105,000 daily boardings once in service, including 52,000 daily boardings by new subway users served by the line's differing path from the defunct Line 3 Scarborough RT.

The transit agency boasts that the controversial extension will bring 38,000 more people and 34,000 more jobs within walking distance to transit, while reducing annual greenhouse gas emissions by 10,000 tonnes.

While the subway is predicted to provide a huge boost to the areas served, other businesses expropriated to make way for the massive infrastructure investment have spoken out against the project.