Learning spanish once you are in mexico is necessary. It will be easier to learn, once you are there and immersed in the language. Many people in mexico can speak fluent english, and a lot of people can speak a little english-and are happy to be able to practice with a native english speaker; but a lot of people don't speak any english at all, so you can't expect to get by with english. it will be very helpful for you if you already know common verbs and vocabulary, how to ask for directions, and what you need, and how to listen to the response, numbers, and what to say when someone asks 'Como estas'?
I found that often people in mexico were very patient with my thick english accent and slow speech, and were helpful at providing the vocabulary i was searching for. If you go down not knowing a single word of spanish, you will learn to speak very basic conversational spanish that will get you by, and you will keep improving- within a week to a month. Just be open and friendly and talk talk talk to everyone, even if it feels like you are having the same conversation over and over again. You'll learn new words everytime you talk to someone! In mexico it seems like the trains are full of riders, so you should be aware of your surroundings at all times. It is easy to get overwhelmed, and feel out of it when you are struggling just to understand how much your orange juice cost. Learning languages through total immersion is intense and exhausting, but always fun! Also make sure you get out there and party, especially with people your own age. Mexican people of all ages love to party and have fun. If you like to sing and play instruments, drink, get loud and crazy, or dance, you will have a great time. Even if you don't like partying, in mexico you will love to party! So make sure you're not just keeping to yourself, if those trains work out for you. Get yourself out there and join the fiesta!