This map from RTD shows the new B Line route between downtown Denver and Westminster, which opens Monday, July 25, 2016.

(Courtesy RTD)

A commuter rail line will connect Denver to the northern suburbs for the first time beginning Monday.

The B Line is a 6-mile nonstop ride between Union Station and Westminster that will take only 11 minutes.

Regional Transportation District spokeswoman Tara Bettale said the B Line will eventually extend to service Boulder and Longmont, but that won’t be ready for decades.

“Right now our projections are showing 2040 approximately,” Bettale said, adding that the expansion relies on sales taxes approved by voters in 2004. "We don’t have an exact timeline."

RTD will also have to devise a plan to share tracks with freight trains to complete that expansion, Bettale said. Commuter trains with diesel engines will also run on the tracks from Westminster to Longmont. RTD currently operates only light rail and electric commuter trains.

Rides on the B Line will be free all day Monday.