Making the trains run on time is proving a difficult task for Metro officials, as a signal problem that’s languished for more than two years continues to stall railcars along the city’s most heavily-used transit corridor.

Relief, however, is expected to come as temperatures cool and officials replace defective axle counters that are the main cause of the problem.

Exacerbated by high temperatures, the axle counters installed by HRT – builder of the three most recent rail segments – do not reset or properly count passing trains. The miscounts keep the system from letting trains pass because they doen’t give operators a signal to proceed.

So trains sit, as Metropolitan Transit Authority officers and operators manually confirm the intersection can be passed safely. Delayed trains mean people wait at stations and train trips take many minutes longer than scheduled, especially along the Red Line through downtown along Main Street. The problems sometimes cascade north of downtown and into the Texas Medical Center, but are generally relegated to the downtown core where the light rail lines cross.

“The frustrating thing is it is not predictable,” said Paul Conti, 49, who rides the train from Midtown to downtown daily. “One day, it’ll be fine. The next it takes you ten minutes to get two stops.”

There are some consistent trends of when and why delays occur, Metro CEO Tom Lambert said. In the mornings, Lambert said Red Line trains in July were on time more than 90 percent of the time. A train is considered late if it arrives more than five minutes after its scheduled arrival. Between noon and 6 p.m. – when Houston is sweltering – on-time performance dropped to 39 percent, before improving somewhat in the evenings as temperatures dropped.

Overall, the Red Line had an average on-time performance of 70.8 percent, while officials’ goal is 95 percent of trains to arrive on time.

Addressing the issue means replacing the axle counters, which officials have known would pose problems since mid-2014. Metro and HRT have spent more than two years testing various replacement options, finally settling earlier this year on a product that works.

HRT chose the defective product, Lambert said, so they are covering the replacement with no increase in their contract with Metro to build the northern extension of the Red Line and the Green and Purple lines that opened in May 2015. Metro is also keeping track of the cost of sending workers to reset defective counters and other costs, Lambert said, which will be passed on to HRT.

Installation of the new counters could start in the coming weeks.

“The first priority is downtown where you have the Main Street line where it intersects with the east-west lines,” Lambert said.

Crews hope to have all of the new systems installed in downtown along the Red Line and where the Green and Purple lines share tracks by Nov. 13. Workers will then move to the Green Line, completing that work by Dec. 18.

Also in December, officials plan to be running trains over the new overpass at Harrisburg, extending the Green Line to Magnolia Park Transit Center. Lambert cautioned the extension of rail service could be affected by the axle counter replacements.

The final axle replacements would not happen on the Purple Line along Scott and the extension of the Red Line north of downtown until March and April, respectively.

Though the delays have frustrated some commuters – among them Metro board member Christof Spieler, who railed against the delays in July – Lambert said he is not convinced riders have abandoned the trains.

“Our boardings are up,” he said. “I think it is something we have to pay attention to, but I am not seeing it in the numbers.”

Metro changed its bus system in August 2015, which focused on transfers onto key rail stations, which could affect ridership comparisons.

It is expected officials will need time to adjust and confirm the system, which relies on constant communication with points along the line, is operating properly.

“It won’t be installed and it goes away overnight,” Lambert said.

As transit and city officials begin the last steps before thousands of visitors descend for Super Bowl LI in February 2017, Lambert said he does not expect the delays to be affecting as many trains. For one, Houston will get relief from the heat, and most of the downtown work will have been completed and tested.