The NTSB released its safety recommendations for the Metro-North Railroad Wednesday following an 18-month investigation.

The group was tasked with figuring out what went wrong in five Metro-North accidents that injured hundreds and resulted in the deaths of four passengers and two employees.

The railroad was advised to improve training and communication for its workers. It was also advised to address infrastructure problems that were ignored for years on the rails.

The issue of sleep apnea among engineers was brought up as well. Sleep apnea is said to be the reason why an engineer fell asleep on Dec. 1, 2013 while at the controls and going more than 80 mph around a bend in the Bronx. That deadly derailment claimed the lives of three people from the Hudson Valley. The engineer's sleep apnea was not diagnosed at the time. Now, medical screenings will be required for those operators.

The NTSB did commend Metro-North for already moving forward on several safety-related issues.

"We will continue to work on improvements, such as installing cameras on trains, fatigue management programs and vastly improved track inspections," said NTSB acting Chairman Christopher Hart. "Our dedicated employees realize that safety is the first priority in everything they do."