After visiting several rail sites in addition to this one, I was truly surprised to see how much misinformation was being tossed about. While I expected such misinformation, and in many cases crass ignorance to crop up on the general discussions of numerous mainstream media websites, and the same misinformation to come from the mouths of various news reporters, I certainly did not expect it from the rail(fan) community, which generally-speaking is more savvy, and if anything knows how to find essential data, such as track layouts, location of interlockings, speed limits and restrictions, types of operations, etc., etc.
For starters, the ill-fated Metro North Hudson Line train departed Poughkeepsie, NY, at 5:54 AM, making stops at New Hamburg, Beacon, Cold Spring, Garrison, Peekskill, Cortlandt Manor, Croton-Harmon, Ossining, Tarrytown, Yonkers, 125th St.-Harlem and Grand Central Terminal. Just north of the derailment site, there is a (major) interlocking, which in addition to allowing Amtrak trains to switch from/to the Metro North 4-track mainline, permits all trains to operate in different directions depending on traffic requirements.
When all southbound trains (both Metro North and Amtrak and the occasional CSX freights) approach that interlocking, they are required to downgrade their speed to a maximum 30 mph. In the case of Amtrak trains, the resume-speed indicator applies when the train has sufficiently cleared the Spuyten Dyvil bridge that runs over the Harlem River (at its confluence with the Hudson River). In the case of Metro North trains, the speed restriction continues from the aforementioned interlocking well past the Spuyten Dyvil rail station because of the numerous S-curves on that portion of the route. Indeed, the route between the Spuyten Dyvil interlocking to the junction of the old (and now abandoned) Putnam Division of the New York Central is one of high curvature and tight clearances, mandating various speed restrictions.