I was riding this train back to Tokorozawa the other day. I was riding on the very front train car and it was completely empty. I've noticed that the train cars on the end of either side of the train tend to be less crowded.
Anyways, I can't imagine why I didn't notice this when I got on...but it was a design train!
These are the famous characters of the manga and anime series Galaxy Express 999 (銀河鉄道999). It seems that the creator of the series, Leiji Matsumoto (松本零士) grew up along the Seibu Line, which led to this collaboration with Seibu Railways and Nerima Ward.
The blond lady you see below is Maetel, one of the main characters (and one of the only characters I actually know of) in the series. The design really stood out and everyone passing by was staring at the train. Some were even took photos, like me!