The 257s are only on selected peak services - most are 15 car E231/233's with double deck Green cars.
I had made a suggestion for the 257 in the loco request thread but then moved it to here, so basically this was a request for the 257 specifically with this route, not any EMU - I probably didn't make this clear in the OP. But also, isn't the Shōnan service all 257s, with the other rolling stock being used for non-Shōnan services on this line?
Ah I see where I'm confused - I'm thinking of the Shonan-Shinjuku-Line regionals*, I see they've upgraded the Shonan Limited Express from a Home Liner Peak only which used 185s into a Limited Express with 257s from last month. That said there are only 3 per day into Shinjuku via the Shonan-Shinjuku line and 2 Outbound - most go into Toyko via Shinagawa *They normally run from Tokkaido/Yokosuka Lines, then via the Yamanote Freight line through Shibuya/Shinjuku/Ikebukero to pickup the Tohoku main line to Omiya then either to Takasaki or Uetsuomiya