The boom arm of a truck-mounted crane smashed into the front of a C-Train and then swung out over traffic on Crowchild Trail, witnesses say. (Submitted by Dean Osmack)

A miscommunication appears to have caused a mobile crane to hit a light rail transit train in northwest Calgary on Wednesday, injuring six people and snarling traffic in the area.

Witnesses told CBC News the crane section of an Enmax boom truck was hanging above the C-Train line and swung toward the oncoming train at 10:18 a.m. MT, smashing into its front window.

The driver of the C-Train was taken to hospital in serious but stable condition. (Faiz Jamil/CBC)

"I just heard the smash and everybody was laying on the ground," said Sebastien Gariepy, 27, who fell off his seat. "Everybody screamed all at once."

The impact derailed the three-car train between the Brentwood and Dalhousie C-Train stations, throwing many of its 30 passengers to the floor.

The crane then swung out over traffic on Crowchild Trail.

"It was actually quite quiet, which is probably a good thing because cars were swerving to miss the crane," said Angela Principalli, who was driving by at the time.

The C-Train driver was rushed to hospital and is in serious but stable condition, Calgary paramedics said.

Bleeding driver checked on passengers

Gariepy said he went to help the driver, who was lying on the control panel with broken pieces of glass on top of him.

Paramedics took six people to hospital, including the C-Train driver and a 10-year-old girl. (Faiz Jamil/CBC)

The man suffered cuts to his head and was bleeding badly from his hands, but still walked into the first car to make sure everybody was OK, Gariepy said.

Four other adults and a 10-year-old girl were also taken to hospital with minor injuries, said Stuart Brideaux, spokesman for Calgary Emergency Medical Services.

Other passengers were moved to a transit bus and treated for minor cuts and bruises sustained when they fell, he said.

Calgary police, Calgary Transit, Alberta Occupational Health and Safety and Enmax are all investigating.

Miscommunication on timing caused incident

The five-person Enmax crew was scheduled to do some routine maintenance at 10:30 a.m. MT that would have lasted less than half an hour, said Gary Holden, president and CEO of Enmax, the city-owned utility.

The parked Enmax boom truck was scheduled to do routine maintenance work. (Faiz Jamil/CBC)

Calgary police and fire officials said the crew was waiting for the track to be shut down. But Holden said it's normal in such cases for Enmax staff to work alongside trains that have been asked to slow down.

"The work was to start at 10:30 and the trains were going to be operating through that area at 20 km/h. But that's irrelevant as it appears the workmen started earlier than that," he told CBC News.

"The big issue here is it looks like a miscommunication on timing."

Holden said the workers involved were debriefing Enmax investigators on the incident.

The crew appeared to be putting the boom into place to start their work when the accident happened, said Calgary police Sgt. Joerg Gottschling.

"There was apparently a miscommunication as to when that boom was allowed to be over top of the tracks between Enmax and the C-Train. We at this point don't know who's at fault for the miscommunication," he said.

Traffic slowed to a crawl on Wednesday as the southbound lanes of Crowchild Trail were closed for the investigation.

Trains will alternate using the unaffected northbound track in both directions, as damaged parts of the southbound track are repaired, said Calgary Transit spokesman Ron Collins.