If you need to get from T1 to T2 or vice versa at MEX, there is a train that loops you around the airport. It runs outside the secure area, so if you are making a connection you will have to go through immigration and customs formalities, meaning you must fill out the forms you get on the plane, wait in lines, get your stuff x-rayed on the way out, and do the green-light/red-light button-pushing business (which determines if you get a secondary inspection). On the bright side, if you’re a passport stamp collector, it’s another stamp for ya. Admittedly, it’s kind of a pain especially if there is a line at passport control, so allow yourself at least 30-45 minutes from leaving your plane to arriving at the other terminal, more if you need to wait for bags.
To access the train station you will need to show proof that you have a flight departing from the other terminal. Despite some reports that only carry-on bags are allowed, I saw many people with large pieces of luggage. Just be aware you will have to navigate escalators, stairs, and ramps. If you’re arriving at T2, follow the signs to exit to the city, head upstairs and all the way to the right. The train station is at the other end of the building. In T1, the station is located in the midpoint of the terminal between the international and domestic sections.
If you’re coming from the city, this is also an alternative to the subway-to-T2 walk I wrote about earlier, since T1 is much closer to a Metro stop, but I have not done this so I can’t comment on how well this works. (Anyone with any experience?)