Just days after a train derailment killed four people in the Bronx borough of New York City, the Federal Railroad Administration has written to the Metropolitan Transportation Authority asking them to make a series of safety enhancements.

The Metro-North Railroad train left the New Haven line track as it turned a 30-mph curve at 82 mph on its way from Poughkeepsie to Manhattan's Grand Central Station at about 7:20 a.m. EST on Sunday, resulting in the deadliest railroad accident in New York in more than 20 years.

In a letter Tuesday to MTA Chairman and CEO Thomas F. Prendergast, the Department of Transportation and the FRA said that the agencies continue to have "serious concerns" about a recent series of accidents that have resulted in five deaths and 129 injuries to Metro-North employees and customers over the past six months.

Joseph C. Szabo, head of the FRA, called the MTA's recent safety record "simply unacceptable." Szabo said that the series of incidents have had "tragic and catastrophic consequences" and have "eroded the public's confidence in the safety of the railroad transportation system." 

Szabo said that a federal team has been working closely with Metro-North and the MTA to enhance safety, but he added that "immediate corrective action is imperative."

While New York Gov. Andrew M. Cuomo has called for a safety stand-down -- which would allow railroad employees to take a "time out" from their normal duties for an in-depth safety discussion -- the DOT said it doesn't believe this is enough to address the safety needs of the track. The FRA has said it will be in touch with the MTA in the near future regarding further safety options.

The FRA said it expects the MTA to reply to its letter by Dec. 6. 

The latest derailment has put a spotlight on the track’s previous repair history and particularly on one open investigation that states the track was found on May 15 to be faulty, just two days before a crash resulted in the derailment of a train near Bridgeport, Conn., and injured dozens.

According to a Capital Needs Report from the MTA, Metro-North will need $3.4 billion between 2015 and 2019 to operate effectively.