NEW YORK — U.S. transportation officials ordered the Metro-North Railroad on Friday to quickly overhaul its signal system and temporarily put an extra worker in the driver’s cab on some routes that have major speed changes, including the one where a commuter train derailed, killing four people.

The emergency order by the Federal Railroad Administration was a reaction to Sunday’s wreck in the Bronx, where a train flew off the tracks after hitting a curve at 82 mph, nearly three times the 30 mph speed limit. The lone train operator told investigators he nodded at the controls and didn’t apply the brakes until it was too late.

There was no system in place to ensure he didn’t miss the spot where he had to slow down as the curve approached. Engineers are required to know speed restrictions by heart, but there are no flashing lights, or even signs, to remind them to decelerate.

The order gives the railroad until Dec. 31 to provide a plan and target dates for modifying the existing signal system so trains will automatically slow down in places where the speed limit drops by more than 20 mph.