Houston's light rail system is fully open, but closing out a complicated rail car purchase that nearly derailed the new lines remains a challenge for transit officials.

Metropolitan Transit Authority officials continue withholding $12.9 million from CAF U.S.A. - the builder of the vehicles - as they debate the amount of liquidated damages owed because of delays and delivery of railcars that were overweight, leaky and halted by faulty axles, among other problems.

When those discussions could conclude and what sort of damages Metro could receive is uncertain, transit agency CEO Tom Lambert said.

"We are not there yet," he said. "We are going to continue to work with CAF, address the issues and go from there."

In the meantime, the Metro board on Thursday extended a contract with Parsons Transportation Group, an engineering and design firm, for oversight of the CAF purchase. The extension carries the contract beyond its previous expiration in May to April 2018 and adds nearly $700,000 to the contract, which has already paid Parsons $29.6 million.

All 39 of the new light rail cars purchased are available for service, and carried a higher-than-normal number of passengers because of the Houston Livestock Show and Rodeo.

All of the cars, however, also have a handful of fleet defects that CAF will have to correct, said Scott Grogan, Metro's senior director of rail operations.

Series of problems

Defects are not unusual in railcar production, and CAF has had some notable failures in other cities lately.

In December, CAF-built vehicles were shelved in Cincinnati for various mechanical issues. Washington D.C.'s transit system - far larger than Houston's - has had widespread maintenance issues with older and newer trains.

Though an axle issue parked some of Houston's trains more than a year ago, the manufacturing defects do not compromise safety. The axle issue remains the most pressing and complicated defect to remedy, officials said.

CAF is also working on 18 modifications to the trains, related to seven identified defects. A problem is considered a defect when 10 percent of the trains have the same problem, officials said.

"All of these that have been identified have an engineering resolution," Grogan said.

He said CAF maintenance can be performed at Metro's maintenance garage, without disrupting rail service. Trains will be brought in one at a time as parts arrive.

"We will go through that process 39 times," he said.

Metro's preliminary schedule calls for all but two of the modifications to be made by the end of the year.

Replacement of a resilient wheel that distributes power to the train's axles is expected to take until February, while the broader axle installations will take until March 2018. The first batch of new axles is not expected until July.

Light on the horizon

The axle problems are just the most recent in years of struggle with the railcars. The bleak history has fueled critics and frustrated even rail supporters.

"I love Metro," said Chuck Baiul, 41, who lives in EaDo near the Green Line and often rides it. "But boy, does it seem like they step in it. Things take longer and cost more than they should."

In 2010, Metro risked losing its $900 million in federal funding to expand the Red Line and build the Purple Line when the Federal Transit Administration determined CAF wasn't complying with Buy America requirements by doing too much of the work abroad. CAF U.S.A. is a subsidiary of the Spanish train-builder Construcciones y Auxiliar de Ferrocarriles.

Work was moved to a CAF plant in Elmira, N.Y., which was slow in filling Metro's order.

By 2013, as the first car arrived, the builder was months behind schedule and quality concerns arose. Spanish officials with the company personally visited Houston, reassuring Metro officials and pledging expansion of the Elmira plant to accelerate delivery.

Things proceeded until late 2015, six months after most of the expanded light network opened, when the axle issues parked 14 of the cars so officials could assess the problem.

Buoyed by Super Bowl

The cars are only part of the stumbles related to the rail lines that Metro has raced to correct. Axle counters along the line led to delays in service for months, dropping on-time performance, especially on the Red Line, which represents most rail trips.

Timing has improved significantly since a blitz of repairs prior to the Super Bowl held last month in Houston. In January, the Red Line posted its highest on-time percentage, 92.6 percent, since November 2015. For many of the months between, fewer than 80 percent of the trains arrived on time.

Officials said despite the lingering issues and unresolved matters, the system is carrying people and growing. Buoyed by heavy use for the Super Bowl week, light rail weekday ridership was 2 percent higher in February, compared to the same month last year.

On Saturdays and Sundays, use was increased 12 percent and 13 percent, respectively.

"This isn't limiting our ability to provide service," board member Christof Spieler said of the railcar repairs.