Imagine staring into the blue portal as this happens. Through the trolley windows, you see the piston, approaching you at high speeds. It hits the trolley, pushing it at the same speed through the portal. Under A, the trolley has to suddenly slow down as it passes through the portal, but that results in the back end moving faster than the front, crumpling the trolley into a pancake.

Then imagine me tackling you out of the way because the answer is B and you're staring into the blue portal.