Streety McCarface
Senior Member
Toronto subway trains can apparently go down to track radii of 250' (77 m)Narrower trains can make tighter turns, and that adds more routing options, particularly for the sections that run at the surface level.
https://www.ttc.ca/TTC_Business/Mat...PandS/Addenda/ADDENDUM NO. 1 - P31BE14063.pdf (1.2.1)
NYC's (I presume for IRT lines) is 29m, less than half that of Toronto's.
That being said, these are both extreme conditions, and no matter how small the allowable radius is, speed will always be the limiting factor. Even though subway trains go really slowly around the union station curve (which is at a turning radius of about 100m, and the smallest radius any train can really travel at), trains are still going relatively fast (25 km/h) compared to a streetcar at a loop (<10 km/h).
It's worth noting that the maximum turning radius along the serviceable Canada line is around 175m, and even along those curves, trains are limited to 35 and 45 km/h. Despite nearly doubling the radius, speeds didn't increase that significantly.
Increasing the cant isn't particularly useful either since it dramatically increases cost and complexity.