(Claritza Jimenez/The Washington Post)

It’s a slightly comical transportation system in the bowels of the U.S. Capitol that few Americans know exists: the Senate subway system. Not subway like Metro — but two sets of tracks that carry underground trains ferrying lawmakers from Senate chambers to their office buildings, less than a third of a mile away.

And it’s the unlikely backdrop to the tumultuous Capitol Hill legislative goings-on of the past seven months.

The subway and its adjoining no-frills, fluorescent-lit station platform have long been a gathering place for the swarm of Capitol Hill journalists, aides and lobbyists who aim to pounce on senators as they disembark from trams or hitch a ride in the same car as a fellow lawmaker, hoping to bend an ear for the 90-second ride from one station to another.

But at a time when Congress has all but abandoned regular order — with legislation crafted in secret, public hearings placed on the back burner and pivotal actions determined by razor-thin vote margins — the transit-station jockeying has taken on new levels of intensity and importance.

Stand at the bank of trains long enough and you’ll get a momentary reading on the state of American politics: There’s former House speaker Newt Gingrich, setting off a flurry of speculative tweets when he’s spotted disembarking from a train to the Capitol. There’s Sen. Jerry Moran (R-Kan.), jumping over the live tracks to ditch a gaggle of reporters seeking details on the Affordable Care Act repeal vote.

In the Senate subway, Sen. Susan Collins (R-Maine) answers questions from persistent journalists on a day when the Senate health-care bill is the topic of the day on Capitol Hill. (Melina Mara/The Washington Post)

There are warnings from the Senate Media Gallery that the subway platforms are too crowded with the hordes of journalists seeking reactions to President Trump’s latest tweets. A lobbyist, waiting for one of the trains, turns to a slightly bewildered-looking police officer.

“Is it me,” she says, “or are things really crazy here today?”

And there, in the middle of the night, minutes before a climactic vote last month on the repeal of Obamacare, Sen. John McCain (R-Ariz.) has a heart-to-heart with Sen. Chris Murphy (D-Conn.) as they make their way from the Russell Senate Office Building to the Senate chambers, a conversation so meaningful that Murphy later said he plans to share it with his grandchildren.

The subway tunnels snaking underneath the Capitol have always been busy, said Donald A. Ritchie, the Senate historian emeritus. But of late, the system has reached new levels of stardom.

“It’s a little like Times Square down there now,” Ritchie said.

The feverish atmosphere may be new, but the underlying infrastructure is not. The need to build an underground Capitol transportation network came more than 100 years ago, when new office buildings were being built beside the Capitol to meet the demands of lawmakers seeking their own office space. To win over the senators miffed at the prospect of exile in an adjacent building, architects came up with a compromise: The government would build a transportation system to ferry lawmakers back and forth to the Senate for casting votes, a chore that sometimes takes place several times per day.

With her daughter on her lap, Sen. Tammy Duckworth moves through mobs of journalists in the Senate subway to vote on the Senate floor. (Melina Mara/The Washington Post)

The tunnel to the offices first featured electric Studebaker automobiles; later, officials switched to trains on tracks out of concern that a lawmaker would one day get mowed over by a careering car.

After multiple rounds of expansions and upgrades, there are now two types of trains on the Senate side: an open-air tram to the Russell building driven by operators who ping-pong back-and-forth all day, and a Disneyland-style driverless train that runs from the Capitol to the Dirksen and Hart office buildings.

Back when the tunnels were first built, such an investment in infrastructure for a rarefied few seemed excessive.

And now, to many, it still seems like an excessive expense. But Ritchie defended the system.

“If the Capitol had been designed as a 60-story building, you’d have a bank of elevators and you wouldn’t be surprised,” he said.

Many of the design changes­ over the years have reflected shifts in Congress’s cultural sensibilities. After Margaret Chase Smith (R-Maine) joined the Senate 1949, plexiglass shields were installed on the open-air trains. (Gusts from the 15-mph train rides mussed her hair to the point that she sat with her head ducked low in the cab.)

And the newer set of cars were designed for accessibility, which helped accommodate lawmakers such as former senator Max Cleland (D-Ga.), a Vietnam War veteran who had lost both legs and his right forearm and used a wheelchair during his time in the Senate.

Some politicians’ refusal to use the subway served as a political message. Former senator Mike DeWine (R-Ohio) refused to take the train in a protest against government waste and forbid his staff from riding too. Hard-charging former senators Hillary Clinton (D-N.Y.) and Kelly Ayotte (R-N.H.) were both known for skipping the trains to power-walk through the adjacent tunnel walkways.

And occasionally, the Senate subway has been a place of confrontation. In 1950, as Smith prepared to give a speech on the growing risks that McCarthyism posed to freedom of speech, she was approached by none other than Sen. Joseph McCarthy (R-Wis.) as she boarded the train to the Capitol.

“Margaret, you look very serious,” Smith later recalled McCarthy saying. “Are you going to make a speech?”

“Yes,” she responded, “and you will not like it.”

According to Smith, McCarthy used the rest of the subway ride to make threatening remarks in an (ultimately unsuccessful) attempt to intimidate her from making the speech.

But for the most part, the subway is a place of bipartisan goodwill.

Sen. Al Franken (D-Minn.) recalled that his first meeting with Sen. Charles E. Grassley (R-Iowa) took place on a train. In his memoir, Franken called the transit run-in “a real meet-cute” — Grassley’s opening line: “You look just like you look on TV!” — and the subway-train bonding session laid the groundwork for extensive legislative co-sponsorship.

Rushing out of the Capitol and back to his office last month, Sen. Benjamin L. Cardin (D-Md.) said that he’d occasionally time his subway ride strategically to catch a colleague and talk policy. Once, he said, he used the 90 seconds on the subway to persuade the chairman of the Senate Judiciary Committee to move a federal judge.

“We’ve gotten some deals done on the train,” Cardin said. “I mean, you’re looking for somebody, and you know you have a captured audience for about a minute. For the Senate, that’s a long time.”

And Ritchie, the historian, has caught more than one heartwarming senatorial moment inside a packed Capitol Hill train. One time, he said, he stepped into a subway car and encountered a group of senators on their way to vote on a bill — doomed to fail — that would have ceded the District of Columbia to Maryland.

One lawmaker turned to Sen. Bernie Sanders (I-Vt.) and quipped: “Bernie, why doesn’t Vermont take the District?”

“Oh, no, we’re planning to annex Quebec,” Sanders shot back, according to Ritchie.

The banter proceeded among the senators, with more and more preposterous proposals, until Sen. Thad Cochran (R-Miss.) piped up.

“Whatever you do,” Cochran said, chuckling, “don’t secede. We tried it, and it doesn’t work.”

That type of idle joking and banter, Ritchie said, is an important part of fostering civility across the political aisle.

“The real problem is that there’s so little social time between them, outside of the legislative process,” Ritchie said. “Those few 90 seconds on the trains may be one of the very few off-camera moments they have when they can actually joke with each other. You have to make the most of that time.”

Those interactions, however, might be growing increasingly rare. Bipartisanship in Congress is arguably at an all-time low. The senatorial gentility of yore has, at times, given way to the rancor and harsh words of the Twitter age.

And then, there’s the most troublesome shift of all: the advent of the Fitbit.

As Cardin power-walked down the walkway next to the Dirksen-Hart subway line, he admitted that he hardly ever rides the train anymore.

The senator lifted his hand and pointed to the slim black band on his wrist.

“I gotta get my steps in,” he said, as another train zoomed by.