When you say Seibu Railway here, people automatically think of this yellow train.
The first yellow train was made in 1969, when the Seibu Chichibu Line opened. Since then there have been several different types of yellow trains. This one in the photo, the 9000 series, was made in 1993. It is a 10-car train and can be seen on all current Seibu lines.
Although newer trains that are no longer yellow have been developed, with nicer seats and 15-inch liquid-crystal displays, I still like the yellow trains the best.