Thought I'd mention it since there are a few questions about layovers in MEX and the easiest way of getting to the city center.

The new line picks up at both terminals (door 7 in T1, unsure about T2), and goes downtown using its own right of way, so apparently with little interference from traffic. According to the plentiful help staff, it takes about 30mins irrespective of the time of day. Fare is $30, payable by smartcard that can be purchased for $10 at vending machines just inside the terminal where the bus leaves ($ = MXN ). There is a bus every 15mins or so, according to the help staff.

Downtown the bus runs along Republica de Venezuela, and the stop for the Zocalo is on the corner of Republica de Argentina, about 4 blocks north.