Rapid transit to the northwest suburbs might actually come in the form of a bus system rather than a rail line as initially promised to voters nearly eight years ago.

The Regional Transportation District held a public gathering at the Omni Interlocken Hotel in Broomfield on Wednesday night to lay out how fiscal miscalculations could affect the future of commuter service into Denver for communities from Westminster to Longmont.

While the promised FastTracks’ Northwest commuter rail line continues to be an option for RTD, one other is a BRT — Bus Rapid Transit — which at its core is express-bus service into the city.

RTD’s board is expected to pick an option by March 8.

All three options leave much to be desired, critics say, though two of them at least provide the rail line that residents in Longmont and Boulder have been paying for via a 0.4 percent sales tax increase metro-area voters approved in 2004.

“I understand how it all comes about in the costs and projections, but what’s been disappointing is how we pay into something but are the last ones to see anything promised,” said William Salazar, 55, of Louisville. “It just feels like we’re the stepchild.”

RTD projections on sales-tax collections are about 40 percent lower than their initial estimates.

If approved by the RTD board, voters will be asked to approve another 0.4 percent sales-tax increase in November, RTD general manager Phillip Washington told the 100 people at the meeting.

“This is a big question, but we saw it as our moral duty to come to the stakeholders to tell you what we were facing,” Washington said. “If we had just driven on, despite the increased costs, I’m sure we would be criticized for being fiscally irresponsible.”

Two options before RTD delay completion of the Northwest line to 2024, a four-year lag from the earlier-promised 2020. One of those options opens a number of park-n-Rides with interim bus service while delaying by six months six other FasTrack lines.

The third idea is to scuttle the Northwest rail and replace it with a commuter bus system that would use high-occupancy-vehicle lanes along the clogged Boulder Turnpike.

Riled state senators and representatives from the affected areas shot a letter to the RTD board of directors earlier this month.

“RTD made a commitment to provide rail to the Northwest Corridor, and the taxpayers in our communities have been footing the bill with the full expectation that RTD would deliver on its promise,” according to the letter signed by 12 legislators.

David Migoya: 303-954-1506 or dmigoya@denverpost.com