When I am in Mexico City I always carry my Metrobus card with me. This card gets me through the turnstile to take the Metrobus, and I can also use it on the Metro (subway). There are machines at the Metrobus stops where I can load my card with more pesos. I usually put between 50 and 100 pesos on my card when it is getting low. Each ride on the Metrobus costs a whopping six pesos (about 31 U.S. cents), and a ride on the subway costs five pesos (about 26 cents). Transfers between the Metrobus lines are free, but there is no free transfer between a subway line and a Metrobus line. The card can also can be used on the "tren ligero" (light train) that goes to some of the southern areas of the city. However, it's been a long time since I have used the "tren ligero". Between the Metrobus and Metro, I can usually go to wherever I want to go. I also keep a couple of extra cards that I lend to friends when they visit me down here.
In the photo below, the top three cards are the ones that I have been using for myself and for visitors. Each one is different, sort of like commemorative stamps.
When I arrived in Mexico City on this trip, Alejandro told me that starting in 2020, the old Metrobus cards will no longer be valid. You have to buy a new card, which is now available from the machines. So I have been using up all the pesos on my card and the cards that I would lend to visitors. All of them are now down to a balance of zero. I bought myself a new card and loaded it with pesos. The new card is the one at the bottom of the photo. It's colorful but generic. I will have to buy a couple more before my cousin and her friend arrive in less than two weeks.
Alejandro has no idea why everyone is being required to buy new cards. They cost less than one U.S. dollar, but given the millions of people who use the transportation system, that's a lot of pesos going into the city coffers. Yeah, it's probably just to get more money.