Three of those cities with continued commuter rail services are in the Northeast Corridor. In all of those areas, public transit agencies have assumed responsibility for the operations. Those public agencies generally also own the trackage used, except for trackage owned by Amtrak as part of its Northeast Corridor route.
Although commuter rail service has been preserved in those Northeast Corridor cities, not all lines survived. Service was eventually discontinued over various marginal lines or branches. Many of those lines had service during rush hours only. In addition, particularly in New Jersey, some commuter lines had their terminals changed, or were otherwise restructured by connecting together various different route segments. "The Official Guide of the Railways" dated January 1930 is an important resource for this section. This was the beginning of the Great Depression, which subsequently led to the discontinuance of many local passenger trains throughout the country. Presumably few if any commuter trains were added after 1930, only to be eventually discontinued. Generally, a route was identified as having commuter service, if at least some trains were scheduled to enable one to work a normal 8 hour day in the city.
Zoomable Google Maps show these present and past commuter railroads. Shown on these maps are past and present commuter rail routes, along with major stations and terminals, and interlocking towers of the past. As well as at junctions, interlockings have existed on 3 and 4 track commuter lines, where trains could switch between local and express tracks.