Metro's railcars are breaking down far more often than the transit agency would like, causing delays and frustrating riders stuck waiting for trains.

Metro's railcars on average made it only 46,274 miles between breakdowns that caused at least a three-minute delay in 2012, falling short of the agency's target of 60,000 miles, according to new data to be presented to Metro's board on Thursday. Those breakdowns can mean long delays, late trains and packed platforms for riders during Metro's peak periods.

While the railcars' average performance for the year fell short of Metro's goal, their performance improved significantly in the last quarter after engineers fixed a persistent door problem.

The worst-performing cars were Metro's beleaguered 4000 series, which lasted only 26,389 miles between breakdowns -- their worst performance in the three years that Metro has been tracking their reliability.

"Similar todifferencesamong different types of escalators, not all railcars are created equal in terms of reliability," Metro spokesman Philip Stewart said. "The good news is that the [4000 series] makes up less than 10 percent of the total fleet."

Of Metro's 1,104 railcars, 100 are the 21-year-old 4000 series cars, Stewart said.

Metro is considering replacing the poor-performing cars, but it won't be anytime soon.

In 2011, Deputy General Manager Dave Kubicek said he expected to be stuck with the 4000 series for six to eight more years, if not longer.

Even though Metro will get new 7000 series railcars next year, those are needed for the new Silver Line and to replace the 1000 series cars, which federal transportation safety officials say are especially dangerous. A 1000 series car collapsed in the 2009 Red Line crash, killing nine people.

Only one railcar model met the agency's goal in 2012. Metro's newest cars -- the 6000 series -- lasted 71,581 miles between breakdowns.

But overall Metro's railcars improved significantly in the last three months of 2012, soaring above the agency's target in those months, largely because engineers pinpointed the cause of ongoing door problems, the agency said in its report. Door problems are the most common issue with railcars, with brake problems ranked second.

"Engineers figured out a technical solution to mitigate one source of door problems, which was highly effective on half of our fleet," Stewart said.

The fix worked on the 2000, 3000 and 6000 series cars, reducing delays on those cars by 87 percent in November compared with the same month the year before, Metro said.