People do eat bento on long distance trains, but usually do not eat on commuter trains. This is usually scene on Japanese trains.

Both at Tokyo station and Shinjuku station there are a lot of bento sold inside of station and outside of station. You will have so much to choose from, especially on Tokyo station there is a bento shop inside of station which sell bento from all over from Japan.

There are Western item bento box also, but usually decoration and taste tend to be for Japanese taste buds. There are sandwiches and meat (even steak, although steak bento tends to be expensive) sold as bento. At Tokyo and Sinjuku station there will be a lot of selection so I think you should have no problem finding items you like.

Pretty much most bento has label indicating to consume on the day of purchase. It is freshly made and not packaged to last longer. Most bento are made fresh the day of sales, I think that is one of reasons bento is popular in Japan. Even if you buy bento at the end of the day, likely those bento were made early in the morning. But if you can keep bento in a refrigerator then I think it should be fine consuming next morning. A lot of bento will have cooked rice, it is Japan after all, and then placing in a refrigerator will make cooked rice hard. Microwave oven will bring cold cooked rice back to nice fluffy rice, if you have access to one in the morning.

Enjoy bento on a train!

Now I am hungry. :