I'm just about to take a masters as well, and I'll be working part-time in retail. Whenever I've asked people about it, most people say that working up to about 16 hours a week whilst doing a full-time masters is fine, and I found a few who worked 3 days a week and were still fine. It takes determination, but definitely possible! Studying and working will look good on your CV as well, as it shows that you have good time management skills. And like rewt said, use the train journeys as an opportunity to study, I used to study on the train all the time at undergrad and done some of my best work. Good luck! :)