Only example I have heard of Canadian military deployed to mass transit in Canada was in 2022, when the 48th Highlanders Reserve of Toronto did army urban warfare practice at Bay Lower, a ghost station attached to an operating TTC station (but they were just using it to access the closed station).
To be clear, the soldiers were unarmed and live ammunition was not used. This was a training exercise. The only armed people on the Toronto Subway are the local Toronto Police Service. The timing of this was interesting though, as it was just a few months after the Russian invasion of Ukraine.
yeah Metros can be a common site for situational practice for domestic terrorism scenarios
A benefit of having disused but fully functional ghost stations or ghost platforms (like Bay Lower) is that you can do such practice without inconveniencing or alarming passengers. Bag Lower is widely used for filming, practice by Canadian police and fire services, and (as mentioned) the military.