While the city waits on the Downtown Relief Line, the TTC is busily tunneling a six-stop extension of the Spadina line from Downsview station into York region. When it's finished, subway trains will travel outside the Toronto city limits for the first time.

Although the project isn't scheduled to wrap until the Fall 2016, the excavation work is close to 70% complete. Holey and Moley, the two boring machines, recently completed the parallel tunnels that will become the southern section of extension between Finch West and Downsview. Torkie and Yorkie, the machines working the northern stretch, are currently approaching Jane Street just north of the CN tracks.

Here's a closer look at Toronto's only active subway construction site.

Length of subway: 8.6 km
Tunnels bored: 6.2 km
Number of tunnel boring machines (TBMs): 4
Speed of tunnel boring machine: 15 metres a day
Time it would take a TBM to bore the length of the Gardiner: 3 years 2 months
Weight of one TBM: 430 tonnes
Number of GO locomotives needed to balance the weight of a TBM: 3.3
Length of cut and cover tunnels: 2.4 kms
Number of stations under construction: 6
Caisson Piling: 110 kms (length of drilled piles)
End-to-end length of caisson piling*: Union Station to Cobourg, Ontario (via DVP and 401)
Excavation: 1,400,000 m3
Concrete: 400,000 m3 (400,000,000 litres)
Spadina extensions required to fill the Rogers Centre with concrete: 3.9
Precast Tunnel Liners: 54,000 segments (9000 rings)
Precast tunnel rings per TTC vehicle: 3

* Caisson pilings are vertically drilled holes that shore up the walls of cut-and-cover parts of the subway so the surrounding earth doesn't collapse in on itself. The figure above is the total length, laid end-to-end, of all the pilings to be dug during construction.

Chris Bateman is a staff writer at theGentries. Follow him on Twitter at @chrisbateman.

Image: Tom Ryaboi