Shinkansen makes emergency stop in Hokkaido's subsea tunnel | The Japan Times
I see that the underground tunnel that runs between the islands of Hokkaido and Honshu has been opened for decades (since 1988), but the Shinkansen service using it is new.
I don't know: being in a tunnel 54 km long and under the ocean in that earthquake prone part of the world - it would certainly make me nervous to undergo an emergency stop.
Update: the Wikipedia entry on the tunnel explains that it is well below the ocean floor - about 100 m or so. But with the massive shift in the ocean bed during that last earthquake, it may not get wet, but I really wonder if it would withstand the worst.
Also interesting to read that 34 people died in its construction.