I will take a stab at this.

This is my 100% guess and 100% speculation of what the OP is trying to do.

Joban Line local service comes from Mito-Tsuchiura-Toride-Nippori-Ueno-Tokyo-Shinagawa, all Joban Line service terminates at Shinagawa and all Joban Line service originate at Shinagawa.

I am suspecting OP is trying to do is that getting on Joban Line service from Shinagawa, trains are not packed and he can grab seats from Shinagawa. If heading to Ueno from Shinagawa using Yamanote Line, Keihin Tohoku Line, or Ueno-Tokyo Line trains has originated elsewhere where trains will be packed at Shinagawa. My guess is that the OP is trying to get trains with space and available seats. All Keikyu Line trains serving HND terminate and originate at Haneda Airport station, so from HND OP can get seats on Keikyu Line from HND. Transferring to Joban Line at Shinagawa, OP will get seats on the train.

Unless OP is heading to Kashiwa, Toride, Tsuchiura, Mito direction on Joban LIne then I am wrong here.

This is the best guess I can make about what OP is trying to do.