Mexico City has an excellent underground train system — one that reminds me very much of Paris's metro. I first caught the metro late at night (on my first night here, with Jackie), and there were few people on the trains, and plenty of room to move about. This gave me the wrong idea as to what catching the metro would be like during the day!
One piece of advice for anyone planning to catch the Mexico City metro in the middle of the day, wearing a backpack that's bigger than you are: don't do it! Trust me, it wasn't much fun. The trains were so crowded, half the people literally couldn't get on, and just had to wait for the next train (where their luck often wasn't much better). Having a giant backpack on these crowded trains made me very unpopular with my fellow passengers.
Anyway, I survived it, and I learnt a thing or two along the way.