Originally ekiben were sold by peddlers walking along the train in an age when trains made longer stops than now and the windows could still be opened. Now you will find them in the station shops, on the platforms and in smaller quantities in the trains themselves. The containers (with small compartments for different types of food) are often very attractive and designed to represent the particular local dish they feature. Ekiben also include disposable chopsticks, a paper napkin and sometimes even a toothpick. They do not come cheap (about 1,000 yen on average), but usually ingredients of high quality are used, making this a convenient way to sample local food.
In fact, these boxed lunches are so popular in Japan that the large department stores in Tokyo sell them during special “Ekiben fairs.” At such times, tens of thousands of ekiben are sold and staff from the restaurants or food factories making the ekiben come to cook in front of the customers. There are in all about 2,000 types.
The most popular ones are:
Ikameshi (Mori Station, Hakodate Main Line, Hokkaido). A whole squid (ika) is stuffed with rice and simmered in a sweet and salty sauce. Each bento contains two or three pieces of squid.
Toge no kamameshi (originally sold on Yokokawa Station on the express line from Tokyo to Nagano, but this station was abolished because of the new Shinkansen line; the bento is now sold in stations along the Nagano Shinkansen line, as Karuizawa or Ueda). Rice cooked in stock and garnished with small pieces of chicken, shiitake, takenoko, gobo, chestnut and an egg. The fun is in the container: a simple earthenware pot.
Masu-no-sushi (Toyama Station, Hokuriku Main Line). Oshizushi (sushi pressed in a mold) made with trout (masu) wrapped in a bamboo leaf and presented in a box made of cedar wood. This is an old recipe from Toyama. But there is much more…
Hamamatsu is famous for its eel (unagi) so here you get unagi meshi; Fukui is known for its spider crab (zuwaigani), resulting in a delicious kani meshi. Another favorite is kakinoha zushi in Nara: oshizushi with various kinds of fish, for example mackerel (saba), wrapped in a persimmon (kaki) leaf, which gives a special flavor to the sushi. And from Yokohama's famous Chinatown hails a shumai bento, containing shumai meat dumplings and other Chinese delicacies.
Sometimes especially the container is interesting, as the Kokeshi shaped lunch box in Morioka, or the Daruma shape in Takasaki (Gunma), a town known for its Daruma festival. In contrast to the above specialty ekiben which are only sold at specific stations, there are also ordinary types of ekiben, with no link to local food.
The most popular here is the Makunouchi bento. Makunouchi is the interval between acts at a Kabuki performance, and in the Edo period people would take this opportunity to have a quick bite. The makunouchi bento contains rice and a wide variety of fish, meat and vegetables.
Here is a site (in Japanese) with an encyclopedic listing of all ekiben.