A proposed mass transit line that would travel down the spine of Austin along North Lamar Boulevard and South Congress Avenue already is shaping up as the centerpiece of a 2020 bond election.

Even though a vote on a proposed 2020 transit bond is more than a year and a half away, enthusiasm to go big on the so-called Orange Line is building. The route exists only conceptually as part of a set of transit goals known as Project Connect and put forth by Capital Metro, but the Orange Line is being hailed as a transformative mass transit development that could fundamentally alter how Austinites get around the city.

And in the wake of a highly successful $925 million bond election in 2018, Austin Mayor Steve Adler believes projections of registered voter turnout climbing as high as 75% come 2020 will bring success with a large-scale transit bond.

On Monday, Austin residents will have their first chance to get a closer look at the Orange Line and ask questions about it when Cap Metro and Austin transportation staffers hold an open house that begins at 3 p.m. at the Austin Central Library, 710 W. Cesar Chavez St.

Chief among those questions for many will be the estimated cost of the Orange Line, which is projected to run 12 miles.

No one knows the answer at this point. In 2018, the overall cost of Project Connect's myriad of proposed new transit routes was estimated as between $6 billion and $10.5 billion.

To get a better idea of cost, Capital Metro hired the engineering firm AECOM on March 25 to begin design work for the proposed Orange Line. The one-year contract runs $5,082,528, but possible extensions for an additional three years could bring the total cost to $12,048,640.

The firm likely will determine the best way to route the Orange Line through Austin without reducing any existing lanes for traffic. Reaching that goal could mean mass transit above roads, below roads or traveling in dedicated traffic lanes built alongside existing roads.

Conceptual maps of the Orange Line show it running from North Lamar Boulevard at U.S. 183 south to the Drag on Guadalupe Street or possibly Lavaca Street and then on South Congress Avenue south of Lady Bird Lake. The exact route has not been set and figures to be a major source of intrigue as the proposed election nears.

"That corridor is one pretty much everyone has traveled," Cap Metro spokeswoman Amy Peck said. "It is very meaningful to people."

Future extensions could lengthen the Orange Line north to Tech Ridge Boulevard and south to Slaughter Lane, according to Cap Metro officials.

Adler and Cap Metro CEO Randy Clarke have said they are open to a slew of possibilities for building the Orange Line without reducing any existing lanes for traffic. At a recent meeting with the American-Statesman's editorial board, Adler threw out various ideas, including a tunnel under the Drag and a tunnel under downtown Austin.

"It wouldn’t surprise me if it were something like taking cars and putting traffic under Guadalupe and come up on the other side," Adler said.

It's also unclear what type of transit would run on the Orange Line. Neither rail nor buses have been ruled out, and Cap Metro has on several occasions floated the idea of autonomous self-driving buses pulling a string of passenger coaches.

"The mode choice isn’t as important to me as long as we get the … Orange Line out of traffic," Adler said.

The pro-rail advocacy group AURA has questioned Cap Metro's apparent embrace of autonomous rapid transit buses, or ART, while the group's questions about costs and feasibility have remained unanswered. The technology was first deployed in Shanghai in 2018.

"ART is an unproven technology deployed in only a few circumstances," read a blog post on AURA's website before arguing that rail remains the most stable and proven mode of mass transit.