Mexico City freeway cards

Hello all.

I have the crazy desire to drive to Mexico City and try to deal with the freeways there. I'm not looking to be talked out of it.

The freeways there have entrances at odd places that are only accessible if you have a particular type of card. On the East Coast, we have a similar system known as "EZ Pass."

Does anyone know where and how to obtain these cards? Mexico City driving is an "experience" under the best of circumstances and takes a lot of patience and sense of humor (OK, it's a nightmare) if you can't use the freeways.

I'll poke around on the Mexican government's websites to see if I can track it down and will report back if no one knows.