Orange County on Tuesday anted up its share of a $6 million study to examine the feasibility of building a SunRail extension from Orlando’s airport to the Orange County Convention Center and the theme parks.

The unanimous vote for a $500,000 contribution makes Orange the latest public agency to sign on, but additional support is needed. And far more money will be required if the long-hoped project is to become reality.

During the Tuesday discussion, county commissioners Mayra Uribe and Kelly Semrad expressed deep concerns about the weightier future funding.

“I’m very skeptical of the project,” Uribe said, doubting its viability. “I hope that we’ll have much more practical, better vetted, alternative options for funding than just asking the taxpayers to pay more in the future.”

Semrad said she wanted commitments from businesses that would benefit from the rail service.

About a year ago, state officials said expanding SunRail to Orlando’s airport and the region’s tourism corridor would boost ridership at least sixfold in the first year of operation but come at a staggering cost of about $4 billion.

That price tag is likely to rise.

The study, known as a project development and environmental study, or PD&E for short, examines projected ridership, the system’s maintenance, operational costs, environmental impacts, station locations and track alignments.

It could take about two years to complete.

Commissioner Nicole Wilson was more enthusiastic about the prospects of the Sunshine Corridor, saying the region cannot solve its transportation woes by adding more and more highway lanes to a congested road network.

The study must be completed before federal and state governments will agree to provide project funding.

Other area governments served by SunRail have each committed an equal $500,000 share. Seminole County pledged in January, the city of Orlando contributed in February and Osceola County kicked in earlier this month.

The Central Florida Tourism Oversight District is due to consider chipping in on Friday.

Volusia County has not yet taken a vote on funding for the study.

The Florida Department of Transportation — which would oversee the study — agreed last year to provide $2 million in funding as long as governments and other private entities along the route kick in the remaining $4 million.

When SunRail launched more than a decade ago, supporters said the ultimate goal was connecting the train — which now runs 61 miles north and south through Volusia, Seminole, Orange and Osceola counties — to the airport, which handles more than 60 million passengers a year.

The proposed rail expansion would open the commuter train up to more than 100,000 people who work around the airport and theme parks, and officials argue it’s a crucial piece of the long-term success of the train.