Nine light railcars remain parked in Houston because of an axle problem, but Metro officials said they're optimistic the manufacturer is making progress toward a solution.

Metropolitan Transit Authority mechanics noticed last month that wheels on some cars were separating slightly from where they connect to the axle. The agency parked 13 rail cars.

The issue, which officials said never jeopardized safety or operations, drew an immediate response from CAF U.S.A., the American wing of Spain's Construcciones y Auxiliar de Ferrocarriles, the builder of 39 of Metro's 72 railcars.

Since CAF diagnosed the problem, Metro spokesman Jerome Gray said, four railcars have been returned to service.

"CAF has given us a safe, proven, immediate fix which involves new axle sets that require a weekly check on the axle," Gray said. "They are also looking at any potential long-term solutions should the immediate fix not work."

The problem - not the first with the railcars - remains a concern until everyone is convinced it is resolved, Metro board chairman Gilbert Garcia said.

"I am on high alert because I want to make sure things are done right," Garcia said, noting CAF sent engineers to Houston within hours of the problem being discovered. "They are making good progress, but I am not completely satisfied. We need to make sure that those cars that are docked or repaired are put back into service."

CAF will bear the costs of repairs.

Metro's problems with the company date back roughly six years, when CAF's procedures nearly cost the transit agency a $900 million federal grant for violating "Buy America" rules. To satisfy Federal Transit Administration officials, CAF moved assembly and more manufacturing operations from Spain to its plant in Elmira, N.Y.

The first batch of the CAF cars, which entered service in January 2015, were months late in arriving in Houston and faced problems related to their weight and failure to pass certain tests. Metro also had to spend $1 million to lift the railcars for maintenance because the new cars didn't conform to Metro's hoists.

The delays became less urgent when completion of the Green and Purple Lines was delayed. The lines opened in May.

The CAF cars are critical to providing service on the new lines and on an extension of the Red Line completed in 2013.

Riders on Thursday said they were mostly unaware of the railcar problems, saying the newer trains seemed more comfortable than older ones. Rudy O'Brien, 20, said getting his bike on and off was easier on the newer models.

His only complaint, O'Brien said, was that wheels on the train squeal loudly at certain spots on the Green and Purple lines. Metro officials said the squealing was related to the track and not the axle issue, and segments where the squealing was most pronounced were being fixed.

Ridership of the lines is improving. During September, October and November - the last month of ridership available - the Green and Purple lines averaged 7,190 boardings per day. Though still significantly below initial estimates provided to the Federal Transit Administration in 2009 for the Purple Line, the total exceeds goals Metro set last year of 5,927 boardings for both lines in their first year of operations.