Greyhound and three other bus companies have not done enough to help passengers in wheelchairs, a lawsuit filed Tuesday in Denver U.S. District Court alleges.

The Colorado Cross-Disability Coalition and three individuals are asking a federal judge to grant an injunction against the bus companies, ordering them to comply with the Americans with Disability Act.

Greyhound spokeswoman Anna Folmnsbee said she could not comment on the specific litigation but said that the company has several programs to aid disabled passengers.

At issue is the treatment the three individuals allegedly received while riding or waiting for bus transportation.

Tim Coenen, of Englewood, says that during a 24-hour trip from Denver to Butte, Mont., he was allowed off the bus only once, although the Powder River Transportation bus stopped five or six times.

"He was left alone on the bus at times, which was very uncomfortable for him," the suit says.

As Coenen tried to board a returning bus, the driver could not find a key to unlock the wheelchair lift, the suit says. Coenen was left behind to catch another ride.Greyhound arranged both stages of Coenen's trip, the lawsuit says.

The other two complainants reported similar problems with regional bus companies.

"Discriminatory policies or a lack of policies seem to plague the over-the-road bus system operating in Colorado," said Kevin Williams, the coalition's legal program director.

Cited in the suit are Greyhound Lines Inc.; Cusa Parts, LLC, doing business as Powder River Transportation; Burlington Stage Lines Ltd.; and TNM&O Coaches Inc.

The plaintiffs seek an undisclosed amount of damages.