At 4 a.m. Saturday, while most of the region slept — save for a few bar revelers making their way home — the Clarendon Metro station buzzed with activity underground.

Down the stopped escalators, past the locked gates, two dozen men wrestled jackhammers over concrete, cut steel with blowtorches, drilled rock and lugged heavy pieces of rail into place in the track bed as bright work lights shone and dust hung in the air.

The workers were some of the 300 Metro employees working in shifts over the three-day Presidents’ Day weekend to install new switches, modules and other parts on the Orange Line. Metro closed the two stops between the Rosslyn and Virginia Square statoins to put in “guarded No. 8 switches.”

In layman’s terms, those are guardrail-like pieces that go on a track and offer extra protection to keep train wheels in place and prevent derailments.

The National Transportation Safety Board recommended that Metro replace switches throughout the system after a 2007 derailment near the Mount Vernon Square station. Of the 80 people on the train at the time, 23 were transported to hospitals and later released. In its report, the NTSB found that part of the cause of the derailment was not having a guard­rail on the switches and Metro’s failure to follow earlier warnings to make the change.

Metro needs to replace 174 of the switches by the end of June. It had been criticized by the NTSB for not moving fast enough, but Metro General Manager Richard Sarles has pushed for more weekend work so the replacements can be completed faster. After this weekend, only eight will remain.

But who’s counting?

Darvin Kelly is, for one.

The Kentucky native, who did stints at transit agencies in Baltimore and Dallas before coming to Metro, helps oversee track work as a general superintendent. He started his shift at 9 p.m. Friday to make sure everything was ready.

Switches aren’t the only equipment being replaced during the three-day weekend. Crews are putting in new ties, fasteners and fiber cable to improve cellphone coverage on parts of the Orange and Red lines. They also are working on other repairs, such as replacing light fixtures, fixing floor tiles, and servicing escalators and elevators. The Clarendon and Court House stations on the Orange Line and Grosvenor-Strathmore on the Red are closed, with buses shuttling passengers past the gaps in rail service.

Just to coordinate track work requires weeks of planning meetings and two weeks of prep work. Replacing switches, for example, can take 12 to 16 hours, transit officials said, and involves crews working on both sides of the track and hydraulic equipment, cranes and bolt-cutting machines moving in the area.

The noise is so loud that the crews don’t try to talk.

“Many of them have worked with each other for so long they don’t have to talk,” Kelly said. “They know what to do. For these guys, it is in their DNA. It is in their blood to work on rail.”

While the shutdowns and the resulting delays frustrate riders — to the point that some say the system is unusable on the weekend — Kelly says the time is critical for his crews.

“There is so much work to do that it takes every bit of a three-day weekend,” he said.

The old switches have to be taken out and the new ones put in, along with cables, train control modules and other parts. It all has to be tested before passengers start riding at 5 a.m. Tuesday morning.

Crews have worked every three-day weekend as part of Metro’s aggressive push to return the more than 30-year-old system to what Sarles calls a “state of good repair” after years of neglect.

“If we do it right, we won’t have to come back and worry about this for 20 years,” Kelly said.

He said the first time he rode Metro’s trains after years of not visiting the area, he was “sick to his stomach” because the system was in such bad shape.

He just wishes there were more three-day weekends.

Just after 5 a.m. Saturday, Kelly surveyed the work done by one crew and shook hands with the next group of workers coming on duty. He said he’s determined to finish by the June 30 deadline.