Try not to eat or drink while walking and riding trains!
The generally accepted rule of conduct is to not eat or drink on the train and while walking (taking a sip of water or tea is fine). This is because Japanese people tend to be very considerate for other people on the train and streets. They also want to keep the trains and streets clean by not spilling or dropping food.
Therefore, in order to fit in and be like a local, remember to refrain from eating or drinking on the streets and in the trains!