Subway

It is one of the largest and most used subway systems in the world, comprised by 12 different lines that measure more than 170 km and carry 4.4 million people every day. You'll quickly see how busy it is, particularly during the day: trains are often filled to significantly over capacity, and sometimes it will be hot and uncomfortable. Despite the close quarters, it's relatively quick and efficient.

Tickets for one trip with unlimited transfers within the system cost 5 pesos.
Operating hours are from 5AM to midnight on weekdays (starts at 6AM on Saturday and 7AM on Sunday), so if your plans will keep you out beyond midnight, be sure to have alternate means of transport.

Although the Metro lacks informational signs in English, the system was originally designed with illiteracy in mind, so finding your way around should not be a problem. Lines are defined by number but also by a color (most of the time we remember the color, but not the number of the line). Stations are identified by name but also by a pictorial icon that represents that area in some way.
As you enter a Metro station, look for the ticket booth. There might be a short queue for tickets, and to avoid having to always stand in line, many people buy a small handful of tickets at a time.
Once you approach the agent, simply drop some money into the tray and announce (in Spanish) how many tickets you would like (" uno " for $5, " cinco " for $10, " diez " for $50, and so on). You do not need to say anything about where you are going, since fares are the same for everywhere in the system.

The turnstiles won't give you the ticket back. Some frequent Metro users use keycards instead of tickets (this card can be used for the Light Rail and Metrobús systems too), so if you see any turnstiles marked with "sólo tarjeta" that means the ticket reader is broken; just move to another turnstile.

While on the train, you will see a steady stream of people walking through the carriages announcing their wares for sale. Act as if you are used to them (that is, ignore them, unless they need to pass you). Most often you'll see the city's blind population make their living by selling pirate music CD's, blaring their songs through amplifiers carried in a backpack. There are people who "perform" (such as singing, or repeatedly somersaulting shirtless onto a pile of broken glass) and expect a donation.

A few words of warning: there have been incidences of pickpocketing. Keep your belongings close to you; if you have bags, close them and keep them in sight. As long as you are alert and careful you won't have any problems. Women have complained of being groped on extremely crowded trains; this is not a problem on designated women's wagons, or any other time than rush hour. If theft or any other sort of harassment do occur, you can stop the train and attract the attention of the authorities by pulling on alarms near the doors, which are labeled " señal de alarma ."