Highways lining the National Mall.

Gondola rides up to the steps of the Capitol.

At some point in the District’s past, someone thought these were good ideas.

The latest exhibit at the National Building Museum, “Unbuilt Washington,” focuses on architecture plans for the federal city that never came to fruition.

“People who know the city well will find some surprises,” said Martin Moeller, the museum’s curator and senior vice president.

The exhibit takes visitors through plans for the city, from the waterfront and Foggy Bottom to downtown, as well as designs for the National Mall, the Capitol, the White House and Lafayette Square, the Library of Congress, the Smithsonian Institution, the Washington Monument and the various presidential memorials.

The exhibit has been in the works for the past year. After researching various plans for the District, Moeller pulled together items from the collections of the Library of Congress and the National Archives to tell the story of why Washington, D.C., looks the way it does by explaining what it doesn’t look like.

He wants people to look at the plans and realize that “nothing built is inevitable.”

“I hope that people come away with a greater appreciation for the complexities of the design process,” he said.

Moeller points to the Capitol as a prime example. The Dome and its wings are iconic now, but it could have taken several different directions.

“The saga of the Capitol is worthy of a reality show, every twist and turn you can imagine right from the start,” Moeller said.

The exhibit covers the plans for the Capitol pre-War of 1812, when the British burned it.

Originally, Pierre L’Enfant, who came up with the city plan, was supposed to design the “Congress House” and the “President’s House,” but he was fired before he could do so. A design contest for the building was held in 1792. Some of the renderings of the dreamed-up buildings appear in the exhibition. Many encompass the idea of separate wings for the two chambers, but other details vary.

One design from James Diamond shows a mansion-like building with a weather vane in the form of a rooster sitting atop a rounded turret.

“I think the design competition is interesting because it really shows the status of architecture in America in 1792, which is to say, there wasn’t one,” Moeller said.

All of these plans were thrown out because George Washington and Thomas Jefferson didn’t approve of any of them, Moeller said. Eventually, a design by William Thornton was chosen. A 1797 rendering on display formed the basis for the Capitol’s construction. The north wing was completed three years later, allowing the Senate to meet there for the first time in 1800.

The exhibit also covers the variety of plans for the National Mall, including the work of the late Sen. James McMillan (R-Mich.). In 1901, McMillan created the Senate Park Improvement Commission. Inspired by the City Beautiful movement that came out of the Chicago World’s Fair in 1893, McMillan had a team of designers and architects set out to make a new plan for the Mall. The commission is credited with the beginning of how the Mall looks today, but Moeller points out that there are some noticeable differences between what was proposed and what was actually acted on.

Among the ideas that didn’t make the cut: a plaza at the foot of the Washington Monument, similarly structured buildings around the Mall that would result in the tearing down of Smithsonian Castle and enclaves of formal, white buildings near the White House and the Capitol.

Almost half a century later, the next big idea for the Mall was the Capitol Hill Expressway. The plan included two elevated highways that ran down both sides of the Mall. The idea was to get traffic to Capitol Hill faster. Had the highways been built, they would have closed off much of the Mall to pedestrians. According to the curator, it never got past the discussion phase.

“It’s fascinating to read the minutes of the meetings where these things were being discussed because it all sounds so rational,” Moeller said. “These things that we consider inconceivable, they were treating as very logical sorts of solutions to the problems of the day.”

“Unbuilt Washington” will be on display at the National Building Museum, 401 F St. NW, through May 28.