WASHINGTON — The third-oldest rail cars in Metro’s fleet, the 4000 Series, are set for retirement starting next week, the transit system announced Monday. This comes as Metro continues the retirement of its oldest cars, the 1000 Series.

“Their performance is a fraction of what the other rail cars do in terms of their reliability,” said Metro spokesman Dan Stessel.

Stessel said 100 of the 4000 Series cars are on the tracks, which represents the smallest portion of the fleet. They are the least reliable: The 4000 series, Stessel said, only averages around 27,000 miles in between delays, while the more reliable 6000 Series trains travel more than 103,000 miles in between delays.

Stessel added that since February 2016, half of the fleet’s 1000 Series cars have been retired, and that will continue this year until there are no more on the tracks.

Stessel said the end goal is making the Metro system more reliable for customers.

“The vast majority of delays we see these days are the result of rail car reliability issues — not infrastructure anymore,” said Stessel.

In 2016, Metro claimed nearly two-thirds of delays were caused by rail car mechanical issues.

One example of the need to replace older rail cars was seen Monday morning near Foggy Bottom station, Stessel said: Brake problems with a train made up of 4000 and 1000 Series cars led to major delays on the Orange, Blue and Silver lines.

“Getting the 1000s and the 4000s off of the system will go a long way toward driving down delays,” Stessel said.