No alerter in the cab car, or in half the trains operating push-pull:
Doomed Metro-North Train Had Warning System, Just Not in Operator’s Cab --
http://www.nytimes.com/2013/12/05/nyregion/as-metro-north-resumes-service-riders-get-back-to-routine.html?pagewanted=1&hp
"The Metro-North Railroad train that derailed on Sunday included a system designed to warn an operator of a potential accident. But such an “alerter,” which can automatically apply the brakes if an operator is unresponsive, was not in the cab where William Rockefeller apparently fell into an early-morning daze while operating the Manhattan-bound train. It was at the other end of the train."
"Three days after the train tumbled off the Hudson line’s rails in the Bronx, killing four people and injuring more than 70, the Metropolitan Transportation Authority said that an alerter system had been installed in the locomotive pushing the train from the rear, but not in the front cab, where the engineer was positioned, properly, at the time of the crash.
"The train was in a 'push-pull' configuration, common on Metro-North. In such arrangements, trains are pushed by a locomotive in one direction and pulled in the other."
"In effect, trains configured and equipped like the one in the derailment employ the “alerter” system on only half of their runs."
Graphic of cab interior:
http://www.nytimes.com/interactive/2013/12/04/nyregion/Safety-Features-on-Metro-North-Cars.html?ref=nyregion
If the article is accurate about alerter timing, and had an alerter been present, it seems there would have been a window in which the engineer could have suffered a lapse of consciousness without the alerter kicking in:
"An alerter system is designed to sound an alarm after 25 seconds of inactivity, and to apply brakes automatically if an engineer does not respond within 15 seconds."
So while an alerter might have helped prevent the derailment, perhaps not?