Metro is retiring old cars. It matters which ones go first.

Metro is currently replacing its 1000, 4000, and 5000 series cars with new 7000 series cars. The plan is to start with the 1000 series, but the 4000 series cars are the least reliable in the system, so retiring them first might be better for riders.

On average, a 4000 series railcar travels just 25,823 miles between breakdowns, making the series the least reliable in Metro’s fleet, by far. (25,823 miles would be 405 round trips between Shady Grove and Glenmont. An individual car would make several dozen round trips each day.)

In contrast, 1000 and 5000 series cars go nearly twice as far between breakdowns, and the 2000/3000 series, which despite the different numbers is one series, are four times as reliable. The newer 6000s are five times as reliable.

This data is for the period from January 2014 to March 2015, so it doesn’t include numbers for the newest cars, the 7000 series.

A railroad is only as reliable as its least reliable cars. It wouldn’t matter if every train on the Red Line were made up of 6000 cars, except for one 4000 set. If that 4000 train were to break down, it could delay dozens of trains behind it, and sometimes even those going in the other direction as well.

Metro has purchasd 748 railcars from Kawasaki, and its current plan is to replace all of its 1000 series cars with 7000 series ones. This will happen as soon as there are enough cars to provide Silver Line service without drawing down reserves, which is the current situation.

The new cars are coming in at approximately eight cars per month, however once Metro’s commissioning facility is complete, that should increase to 12-14 cars per month. Right now, there are 32 7000 series cars in service, half as many needed to operate the Silver Line without cannibalizing reserves.

The 1000s have been criticized by the NTSB and others because they don’t fare well in crashes. But crashes are relatively rare on Metro. Breakdowns are far more common.

The 4000 series is the smallest set in the fleet, with just 100 cars. Because of the small series size, it will only take about seven months to retire the 4000s. On the other hand, it will take almost two years to retire the 1000s.

Unfortunately, while retiring the 4000s first would reduce the number of breakdowns and delays on Metro, it probably won’t happen due to political pressure to remove the 1000s, which make up about a quarter of the fleet.

Keeping the 4000s in service for two to three more years, as opposed to starting retirement soon, means riders will continue to have commutes marked by delays and offloaded trains.

Regardless of the order in which WMATA decides to retire cars, the 7000s will mean that the three least-reliable series will be off the rails by 2019. Hopefully, the 7000s will be very reliable, like the 6000s have been.

While we don’t have any data on the new trains yet, there’s reason to be confident. Kawasaki has made reliable cars for other transit systems, most notably New York.