The Flatiron Flyer bus service’s success could be its Achilles heel.

Commuters crammed onto the routes connecting Boulder, Denver, Westminster, Broomfield, Louisville and Superior 40 percent more in 2016 than they were the year before along that corridor, and riders are feeling the growing pains.

Ben Heinemann is a frequent Flyer rider from Denver to downtown Boulder every morning for work. He praised its speed and reliability, but there are a few things he wished were different about the service that launched at the start of last year.

“It gets really packed,” he said. “I think it has to do with the timing of classes at CU. If you get one when kids are getting out of class at that Euclid and Broadway stop, it’s definitely crowded with students. It’s fine, but those FF1 buses, especially, get super crowded.”

The Flatiron Flyer — with 57 seats per bus and perks such as bike storage and electronics charging stations — provides six different routes. These range from routes that reach all stations, all day — the FF1 route Heinemann mentioned — to an express service with fewer stops.

While riders said the service was generally on time, fast and otherwise efficient, many noted standing-room-only incidents were not an anomaly.

University of Colorado Boulder student Hannah Fertig, 21, hitches a ride on the Flatiron Flyer a few times a month to get between Denver and Boulder on the weekends.

She’s nearly been left behind in Denver because of long, late-night lines snaking through the station, she said. Arriving what she thought was early enough to catch the second-to-last bus out of Union Station, Fertig was surprised to see a large line of people formed at least 30 minutes before the Flyer was set to embark.

“We were able to make it onto the last bus somewhere between midnight and 1 a.m., and we got back to Boulder super late,” she said. “There were definitely people who didn’t make it on the last bus.”

When riding to the 1st Bank Center around rush hour last year, Fertig remembered a scramble of people trying to give up their seats to older riders forced to stand in the aisles.

“There was barely even standing room,” she said.

The crowds haven’t gone unnoticed, according to Regional Transportation District officials.

“The growth pattern has exceeded our expectations,” said RTD spokesman Nate Currey.

Hitting capacity at certain times of the day has alerted the transportation district that it needs to add more bus frequencies.

“We simply don’t have enough people to drive the buses, though,” he said. “It’s just a challenge of hiring at this point.”

While the six routes seem set in stone right now, Currey said the district is always adjusting for frequency of buses and tweaks the schedule three times a year for that reason.

The next adjustment is in May, but it doesn’t have Flatiron Flyer frequency adjustments scheduled, Currey said.

Kathleen Bracke, manager of a section of the city’s transportation division GO Boulder, is thankful for RTD’s partnership but said there’s more work to be done.

“We definitely see the need for additional service for the Boulder community, both for residents and people commuting into Boulder for work,” she said.

Bracke hears the most complaints about jammed buses heading toward the airport and hopes more services will be added on that route. She also wants to see an express bus between Denver and Boulder during the day, which she said was available before the Flyer existed but now only runs during rush-hour time slots.

Overall, Bracke said the new buses have been an asset to the community, and she looks forward to making them more rider-friendly with RTD.

“The Flatiron Flyer is really dependable,” Currey said. “It’s gratifying to know that people are putting it to use this way.”

Elizabeth Hernandez: 303-473-1106, hernandeze@dailycamera.com or twitter.com/ehernandez