The Acela is, by no straight of the mind... an 'Extreme Train'...
Oh yes it is, I worked on the NECIP and when we were starting we asked the Japanese, Germans, and French to come and offer suggestions, they all said the same thing, "it couldn't be done" but it was do it or no high speed on the North East Corridor.
all other high speed railroads in the world are two tracks one in each direction, exclusively for the high speed trains, one track goes in one direction the other goes the other direction, and mostly all built from the ground up.
the NEC has an average of three tracks, all bidirectional, there are commuter trains, regional trains, freight trains and high speed trains all running 24/7. This runs and works something that no one else has been able to do. and it was done without ever shutting the railroad down.