With an average daily commute of nearly 2 hours in Beijing (China Daily), you might as well use this time productively, right? (Good thing we place our students in apartments close to our school and their internship company!) Why not learn Chinese while you are stuck in the subway on your way to school or work?
Podcasts or audio lessons are a great way to practice your Chinese while you are on the move. Just plugin your iPod or Mp3 player and study anytime, anywhere. The most famous Chinese podcast is ChinesePod. They provide excellent audio lessons in combination with a fully fledged online learning environment. The first 300 lessons are free if you sign up for a trial account. If you want more you will have to purchase one of their study packages.
An alternative is provided by Melnyks, featuring over 200+ high quality audio lessons for free.
Not into all that digital stuff? Just take out your Chinese flashcards or a Chinese pocket booklet! Even better, when you're stuck in the subway why not just strike up a conversation with the your fellow commuters. No better way to learn Chinese than to actually speak it outside of your regular learning environment.