You do no see people eating and drinking in the regular train. If you are on the long distance train like Shinkansen, it is a different story.
There are two reason why you are not welcome to eat on the train.
(1) You will be leaving mess on the floor and seats.
(2) Your food leave smell in the car.
(3) Eating in the places where it is not designed for is not polite.
Unlike Shinkansen, regular train will not have cleaning service for al day, so you should really not suppose to eat anything like what those couple are eating.