Before update 14w06b you could check if a player was in a minecart or not by having a trip wire above the rail (like the wiki shows here.)
Every minecart (chest, tnt, hopper, normal) would go under without activating the trip wire whilst players would activate it if they was in said minecart.

This is no longer the case. Now every cart activates the trip wire even tho it looks like they should go under. Feels like the hitbox of carts has been made taller or just aligned incorrectly. Image shows how the trip wire works (taken from the wiki).