In London on the main lines out of the main termini it's usually 4 tracks abreast with commuter services + stopping services taking one pair of tracks and express + intercity services taking the other pair for in and outbound services. High speed railways (greater than 250 km/h (155 mph)) are a different story usually running on bespoke track.
There are two tracks leading from Manhattan to the west and south of the country. These tracks are shared between both commuter trains from the New Jersey suburbs (population 4.5 million) and Amtrak. Elsewhere, there are still two tracks in places.
The particular problem segment being discussed here does have four tracks, but it has commuter, regional, and intercity services. This is causing congestion problems; IIRC UK HS2 is being built because of a similar problem where they need to get the intercity trains out of the way of fast regional services.
Is that not the case in the Northeast Corridor?