Canadian Pacific Railway says it did everything possible to inspect a Calgary bridge that failed early Thursday morning, leaving six derailed cars stranded on the sagging track.

The company said floodwaters caused the Bonnybrook rail bridge to buckle.

"Despite numerous, vigilant and beyond regulatory inspections of the bridge, one of the four piers which support it experienced scouring," said spokesman Mark Seland.

A frustrated Calgary Mayor Naheed Nenshi lashed out this morning.

"Let me be blunt," the mayor said. "A lot of people have lost their jobs at CP Rail the past year. How many bridge inspectors were let go?

Seland denied later in the day that CP had any fewer inspectors on the job.

"CP today has the same safety policies, practices, and especially inspection personnel that it had last year and even five years ago," said Seland.

He said the scouring occurred at the very bottom of the pier, where the inspectors could not see it.

Brent Laing, CP's vice-president of engineering, said the rail lines had appeared straight and level.

"When you have water up so high, and you can't get a diver in the water … and you can't get any instrumentation in the water, how do you tell if there is a problem below? You look at the rails … if the rails are level, nothing is happening below," said Laing.

The company said it regularly inspects all bridges twice per week according to federal regulations, and those inspections were stepped up because of the flooding.

According to Seland, the bridge has been inspected 18 times since the flooding started.

The cars have been stabilized and officials say they are confident the cars will not fall into the water.

The six cars were part of a train that derailed on southeast Calgary's Bonnybrook rail bridge around 3:45 a.m. MT. They were at risk of falling into the fast-moving Bow River, but officials have tethered the cars together so they will dangle over the water instead of floating down the river if they fall farther.

The bridge initially buckled and dropped about half a metre, but has not dropped any lower since 9 a.m.

Heavy machinery was moved in around noon and crews are in the process of setting up pumps to extract material from the derailed cars.

Five of the Canadian Pacific rail cars contain petroleum diluent, which is used to thin petroleum products, including bitumen from Alberta oilsands, for transporting through a pipeline.

Bridge not among those inspected by city

The bridge was inspected by CP Saturday and the tracks were inspected Monday. The bridge was also scheduled to be inspected today, said CP spokesman Ed Greenberg.

City-owned bridges are safe and have been inspected three times for structural stability following Calgary's devastating floods, Calgary Mayor Naheed Nenshi said at a noon news conference.

The buckled Bonnybrook rail bridge is owned by CP. The city does not have the authority to inspect railway bridges, which are under federal jurisdiction.

That is a long-standing frustration among municipalities and there needs to be a serious discussion about why they are not subject to city regulation, Nenshi said.

"Once this is over, I'll be looking for answers," he said. "It does nobody any good to get angry when we have a crisis situation."

While federal response has been satisfying so far, Nenshi said he was not happy that it took CP officials six hours to return his calls after the train derailed.

He said there is frustration among mayors and cities about a lack of municipal rights to inspect bridges owned by railway companies.

The rail bridge in Bonnybrook dropped more than 0.6 metres in 90 minutes following a 'structural failure,' according to the Calgary Emergency Management Agency. (City of Calgary )

"How is it that we don't have regulatory powers over this, but it's my guys risking their lives over this?" Nenshi said.

Cam Uzeloc, Calgary’s acting fire chief, said earlier today that fire officials were concerned the cars could fall into the river and cause problems downstream at other bridges.

The bridge is 7.5 metres above the water and Transportation Safety Board officials said they were surprised with the situation.

"Don't hear of this happening very often," said inspector James Carmichael. "Bridge structures are usually pretty strong."

Evacuation ordered

The evacuation area around the bridge has been reduced from around a kilometre to 300 metres. There are no homes within the evacuation zone, but several businesses are affected, the city said.

"[The contents] being a flammable product, we wanted to make sure we had people out of the area in case something happened," said Uzeloc.

There have been no leaks identified so far, he added.

The majority of the train made it across the bridge before the incident.

Uzeloc said cables would be attached to the cars remaining on the bridge and that cranes would then be used to remove them.

The incident had a heavy impact on traffic. Deerfoot Trail, a major road in the city, was closed between Glenmore Trail and Blackfoot Trail but has since reopened. A portion of Ogden Road has also reopened.