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 at the controls. It was at the other end of the train.

On Wednesday, three days after the Manhattan-bound Hudson line train tumbled off the 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, 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.

The authority said a “push-pull” model kept the noise and exhaust of a locomotive as far as possible from the halls of Grand Central Terminal, though diesel trains typically switch to an electric mode as they approach. There is also generally no way for trains to turn around there, the authority said.