BART’s shiny new train cars will start rolling out onto the system’s main tracks beginning this week, marking a new milestone in testing for the new fleet, officials said Tuesday.

The agency is in the process of replacing its 669 train cars with 775 new ones, a $2.6 billion effort to improve the reliability of the aging vehicles, increase capacity and improve riders’ experience.

Engineers have been running the first 10-car train on the tracks at night for testing and had planned to see how it behaved alongside BART’s current trains by the end of March or early April, but testing hiccups slowed those efforts. In late April, the test train broke down at Lake Merritt station, snarling the morning commute.

BART had planned to have 60 new train cars in service by the end of the year, but engineers have since revised that estimate to 35. As of Tuesday, that’s still the agency’s plan, which is dependent on this phase of testing, said BART spokesman Taylor Huckaby.

“We do not have a set time frame for testing periods as the overall length is subject to change based on findings discovered during the process,” he said.

BART patrons may see the full 10-car test train, or a shorter version with fewer cars, since BART may use some of the new cars for operator training, Huckaby said.

Only BART employees will be able to board the new train, as it has not yet been certified to carry passengers. The train will run with an “Out of Service” message on it and the platform destination signs will read, “Test Train — Do Not Board.”

The train might stop at stations and even open its doors, depending on the type of test being performed, BART officials said. But it’s significant milestone because it marks the final phase of testing before BART gives the OK to ramp up delivery of “production vehicles,” or the train cars that have incorporated all the changes identified during the testing phase.

Several of those production vehicles are being assembled now and are undergoing climate testing in Canada, Huckaby said.

They are expected to be delivered in September, he said.