The card only access would really not work well because people could follow others in and not everyone riding transit uses ARC. Either a door system less prone to damage or no doors would be better options.
Also I assume any kind of door system needs to have a manual release to avoid trapping someone inside in case of fire or electrical failure. Because of that the doors will always be prone to people just prying them open, even if an ARC card is required to open.
I say we do away with the doors entirely. On most winter days the radiant heater on it's own will be enough to keep people comfortable, but still uncomfortable enough to deter loitering.