Are you sure the E257's run on the Keiyo line? The E257-500 'Boso express' trains (along with the older 255 series) are maybe the ones that run on that line.
On the subject of the E257 series, you can drive this train in 'Densha De Go! Final'
The E331's also run on the Keiyo line.
For the latest Japanese new/refurb etc trains and other 'Gen', there is no better website than http://1st-train.net/ Its updated every day with all the latest photos, simply click the links under the 'Photo-topic' heading.
Here is also a great website showing many (But not all!) Japanese trains past and present, Includes JR and some private railways http://www.uraken.net/rail/alltrain/