Originally Posted by RichardInSF
The way the Narita Express works in the afternoon is that half of the train comes from the south and half comes from Shinjuku (or Ikebukuro). The half from the south is the only part that would stop at Shinagawa. The two halves are joined up at Tokyo station. That's why they have the same number.
Probably the agent said you couldn't switch at Shinagawa because your Narita express seats are in the half not stopping at Shinagawa. IIRC, that would mean your seats were in cars 1-6.
You could actually switch at Shinagawa to the wrong half and then change at Tokyo station but that would be a fairly fast shuffle, you'd only have about 60 - 120 seconds to do it. You cannot access one half of the train from the other even after they are connected, although the conductor can and if you seemed lost, he might take pity on you and unlock the connecting doors if the half you were in was full.
You can't do that from within the train, they are two separate rolling stocks, and the driver's cab is in between. The joining occurs at car #6. When the train is fully joined, and going towards NRT, the
front of the train is car #9 or #12 (depending on whether it is a 9 car or 12 car train, varies by schedule). What you should do is get on at Shinagawa, and just walk towards the
rear of the train until you hit the end. When you get to Tokyo station, quickly get off and walk towards cars #1 through #5 which will be coming in later from somewhere else and walk into the train and take your reserved seat. The ride from Shinagawa to Tokyo is only about 10 minutes, so that's all you will have to stand.