DENVER —

Denver has formally asked the federal government if the University of Colorado A Line can stop blaring its horns, more than two years after the train to the plane started running.

The City and County of Denver, together with the Regional Transportation District (RTD), filed for a joint-waiver on Friday - the first of two steps needed to establish quiet zones, pending approval from the Federal Railroad Administration.

This is just one more stop on the path to full approval for the A Line. Regulators originally required the horns to blare and flaggers to stand at each of the train’s 11 crossings because the timing of the gates didn’t meet government standards – regulators felt they came down too early or stayed down too long.

The FRA approved the timing of the crossing gates last year. The state regulatory agency, the Public Utilities Committee, OK'd the gates in March – allowing for the removal of the flaggers.

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The FRA would not sanction quiet zones until the guards were gone.

This application applies only to the nine A Line crossings within Denver’s limits. According to RTD, Aurora must meet an additional requirement before it can apply for quiet zones at the Chambers and Sable crossings.

Once Aurora finishes that process, RTD will ask the FRA to approve quiet zones for the G Line - the train running from Denver to Arvada and Wheat Ridge, which hasn’t gotten the authorization to move out of the testing phase. The G Line is now testing from 3 a.m. to 1 a.m. and blaring horns through each crossing.

There’s no estimate for how long it could take the FRA to approve quiet zones.

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