Security.

I suspect that the Tram system in Łódź (thanks for the "copy 'n paste" Steve) has been this secure for a looong time.

The only thing that's changed is that, these days, screwing around with it results in arrest, a court case and, probably, a fine. Oh, and a lot of kudos from your mates.

When it was put in, tampering with it would have resulted in some very stern looking people taking you away in the middle of the night, asking you a lot of serious questions in a way that *really* encourages a quick answer. Oh, and your mates will never admit to having ever heard of you ever again.

Deterrence can be just as effective as security.......