It’s been called a quiet success story, a transit model for other U.S. cities to emulate, and one of the most successful bus lines of its type in the country. The Orange Line, an 18-mile bus rapid transit system that runs through the San Fernando Valley from Chatsworth to North Hollywood, opened in 2005 and now serves more than 25,000 daily riders, more than tripling initial estimates.

Los Angeles City Councilmember Paul Krekorian, whose district covers the East Valley and part of the Orange Line route, effusively praises the route’s impact on the lives, and commutes, of his constituents.

“It’s the most successful BRT in the county, and perhaps North America,” he says. “That’s why we’re eager to expand capacity and convenience and make it a better service to accommodate more people.”

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Krekorian is talking about turning the Orange Line into light rail—it’s what many local transit activists have called a “holy grail” of Valley transportation.

In its 12 years of service, the Orange Line has carried more than 74 million passengers and proven there’s an audience and ridership in the region. It’s so successful that it’s often crowded at rush hour, leading Metro to explore an upgrade.

Bolstered by Measure M funding, the transit agency is studying the cost of elevating five street crossings, which will be constructed beginning in 2019, laying the groundwork to convert the Orange Line to rail by 2051. But that far-off shift may happen sooner than expected; earlier this year, a private company submitted an unsolicited bids to accelerate the light rail conversion.

Transportation analysts pose a different question about the Orange Line: Why can’t the country’s most successful bus line finally get some respect?

In addition to the massive price tag (around $1.7 billion, based on a 2015 analysis) and lost opportunity to extend the transit system elsewhere, a rail upgrade by itself would still face the same street crossings and slowdowns, just like the bus. The push to lay down track and replace a successful bus line begs another question: Can a bus line, even with all the right support, ever be enough?

When the Orange Line started rolling in 2005, the novel experiment offered residents who had sought dedicated transit for decades a new commute, and a compromise.

Various proposals to create and fund a Valley light rail system date back to the ’70s. In 1990, the first of a few funding measures was passed to expand the rail system into the Valley, but at roughly the same time, an ordinance was passed to ban a rail right-of-way where the Orange Line is now. The solution was to transform the rail system into bus rapid transit system, a relatively new form of urban mass transit that prioritizes buses on roadways to make them more efficient and rail-like, all without the massive infrastructure investment that comes with adding a train to an already dense city.

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Featuring dedicated-only bus lanes and pre-boarding facilities, the Orange Line helped erase the bottlenecks that typically make buses seems so sluggish in urban traffic. Offering a new transit option, and delivering on its promise of more speed, the Orange Line experiment worked, with buses running 30 to 50 percent quicker than conventional LA buses.

It’s been a success on multiple levels, says Juan Matute, a transportation researcher at UCLA.

“The Orange Line has been an extremely cost effective transportation amenity,” he says. “You could build five Orange Lines for the cost of a light rail corridor.”

What’s even more impressive, he says, is that it’s succeeded despite serious Metro handicaps.

Since the bus shares the road with cars, and its dedicated lane is on the same grade as regular traffic, it has to contend with cars and traffic crossings. The city limits the speed at which the buses can cross intersections to 10 miles per hour, further hampering progress. The Orange Line could also handle 60 buses an hour during peak times, helping space out the capacity crowds that typically ride the line at rush hour, but the city limits the line to just 15 cars. Built to rise above the typical gridlock, the line is often mired in it.

“We don’t want to slow down a few cars, even if it makes a better experience for thousands of bus riders,” he says.

While the Orange Line is the most effective BRT in the U.S., it lags well behind similar systems across the world, especially in Latin American countries like Colombia, where it was first popularized. In fact, the Orange Line is ranked 60th of 106 routes across the world, according to the Institute for Transportation and Development Policy, due to its lack of signal priority.

The great irony of the bus to rail conversion, says Matute and other experts, is that many of the changes needed to make rail more suitable, such as creating raised crossing and favoring rail at crossing, would basically make the bus rapid transit system even better. If the bus line were treated like rail, instead of being replaced by rail, the transit authority could accomplish many of the same efficiency gains at a much lower cost. Transit writer Dam Malouf calls the situation BRT Creep: promising great bus rapid transit, then undercutting it by removing the very features that would make it a success.

“We have to treat the bus more like rail,” says Anne Brown, a PhD candidate in the urban planning department at UCLA who studied neighborhood change around the Orange Line. “We can’t say it’s only a bus and then cut the legs out from under it.”

Councilmember Krekorian says it’s still a pretty long ride across town from the West Valley, and that the system could increase ridership with light rail, which would mean more vehicles, more passengers, as well as grade separations and more speed.

Another argument for rail is that the stations along the Orange Line haven’t seen the same volume of transit-oriented development associated with new rail stations. According to Brown’s research, rising median home values and rents around the Orange Line stations between 2000 and 2013 suggest “gentrification and economic transitions” can result from BRT expansion, even if there isn’t coordinated policy or government support. With more focused policy, the city may have been able to utilize BRT as development tool.

But what if the market for larger TOD projects just isn’t there, regardless of what kind of vehicles are running between stations? Matute says that the laws that apply to transit-oriented zoning and development apply to the Orange Line stations, like they do to other rail stations, but developers haven’t acted because the market just isn’t there. Lots of stations are situated near large swaths of single-family homes. In fact, one of the only serious proposals for this kind of development is where the Orange and Red lines meet, turning parking into mixed-use residential and retail. The conditions that make transit oriented development work in, say, Hollywood, where there are big parcels and a market for this kind of residential development, just aren’t there for the Orange Line.

Another disadvantage of converting to rail is a loss of flexibility. Dropping tracks locks in transit, while buses can always veer off the BRT line to make stops at new stations or points of interest. Matute says major destinations, such as Cal State Northridge, Burbank Airport, and other entertainment districts, could easily be connected to the Orange Line with feeder routes.

“I see the bus/rail distinction more from people who don’t use transit or use it infrequently,” says Matute. “Those who prefer it to be rail generally aren’t using it. There’s a lot of good bus service in LA.”

Krekorian says pushing the rail switch isn’t for “lack of love.” It’s the love of the Orange Line that leads him and other proponents to seek out more capacity.

“The success of the Orange Line opens the door to more BRT and investing it in lots of places,” he says.

Brown and others see the potential price tag—”$1 billion dollars a mile” for rail—and envision all the other buses and transit additions and upgrades that could strengthen the network as a whole. (Metro’s Measure M expenditure plan estimates the total cost of the bus-to-rail conversion of the 18-mile Orange Line to be about $1.4 billion.)

Don’t let the great be the enemy of the good, they say, especially when greatness is so expensive, and a potential cost-effective solution is at hand.

“There should be more of a focus on just treating the Orange Line better,” she says. “It has all the right ingredients. It just needs some Metro love.”