Actually, the Tokaido and Tohoku shinkansens were both JNR companies. Tohoku started 1985 between Omiya in the North and Ueno (a ward within Tokyo). Initially, the Tohoku ran South to Ueno station. Tokyo station was added in 1991. The platforms are adjacent, and I think they are even connected by train. But again, the Tohoku is run by JR East, the Tokaido run by JR Central and the Sanyo by JR West. Some trains run on Tohoku and Sanyo, ie: some Nozomi trains run from Tokyo to Hakata (Note: term to term time is 5:03, with a travel time average speed of 144 MPH). Many of the JR East trains share trackage, but it's been 18 years since I've lived there, and I couldn't map my way around JR East (with regards to Shinkansen anyway).So every line requires a transfer? There's no sharing of track rights? (I thought the government actually still owned the tracks.) The JR web site makes it seem like you can ride one train from Tokyo to Hakata, if you go to "hyperdia" and actually look up the itinerary. (There's also just one JR web site, so they must share some things.)I don't think it's the layout; the tracks are adjacent. See: http://en.wikipedia.org/wiki/Tokyo_StationWhy are there no "through" Shinkansen connecting Tokaido and Tohoku lines at Tokyo? Is Tokyo like Chicago, where it's pretty much impossible to run through trains without major maneuvering? It seems like you should be able to ride one shinkansen pretty much the full length of Japan.
The trains are run by different companies. The Sanyo trains ran to Tokyo before the breakup of JNR. The Tōhoku line started Tokyo service after the breakup.
I have been to Tokyo station many times but just have never ridden anything other than Tokaido line trains.
It's possible for the JR East trains to get on the JR Central tracks, but it doesn't happen. Tokyo Station is a terminal station for many many trains, and it is the common point to change trains. To go from the far North to the far South without changing trains requires a night train (which are slowly fading away).