Which amount of snow stops the trains?
Picture A taken near Kazuno-Hanawa on the Hanawa line? OR
B. Tokyo yesterday snow falling but not collecting on the ground.
And the answer is
A few flakes of snow yesterday and the subways were in chaos with delays and cancelled trains. Most subways turn into suburban above ground trains once they get out of Tokyo so even though they are mostly underground, snow above ground can cause problems. Tokyo is not designed to cope with snow. Because it rarely snows cars don't have winter tyres and any amount of snow invariably causes traffic accidents.
(The photo above is JR rather than subway, the photo below is of subway delays, but if I took a picture in the subway, it would be underground & you wouldn't see how little snow there actually is.)