The Transportation Safety Board has announced it is opening an investigation into a runaway train incident earlier this month at CN Rail's MacMillan Yard in Vaughan, in which 74 rail cars — one of them carrying dangerous goods — rolled away uncontrolled for five kilometres.

The TSB tells CBC News that on June 17 ground crews were using a remote control device known widely in the industry as a "belt pack" to assemble a train when the 72 loaded cars — as well as two empty ones — rolled away uncontrolled.

In a written statement released Tuesday afternoon, CN confirmed a runaway locomotive escaped its control and pulled the cars five kilometres down the track with a crew member stranded onboard. CN also revealed that the runaway cars included "a single dangerous goods load of an ethanol/gasoline mixture." The company says it is co-operating with the TSB investigation.

The cars rolled out onto one of CN's busiest cross-country rail lines, known as the "York Subdivision," the Transportation Safety Board says.

CN spokesperson Mark Hallman told CBC News that the locomotive finally came to rest on an uphill grade near Bathurst Street just north of Steeles Avenue West with help from the train's emergency brakes.

"The movement reached a maximum speed of approximately 29 miles per hour but never exceeded permitted track speed limit and moved on tracks it had rail traffic control permission to be on," Hallman told CBC News.

No one was hurt. CN says employees radioed their rail traffic control centre, which was able to clear area tracks to ensure the runaway railcars did not collide with another train.

"There were no conflicting movements, derailments or injuries, the CN statement reads.

"Note the locomotive and freight cars remained coupled throughout. CN immediately reported the incident to the Transportation Safety Board of Canada. CN conducted its own probe and has taken corrective action to prevent such incidents in future."

Fortunately, the rail line, which curves southward and then east parallel to Steeles Avenue is separated from car and pedestrian traffic by a series of underpasses and overpasses as it crosses Dufferin, Bathurst and Yonge Streets.

Runaway train incidents happen more frequently than most people might imagine.

Earlier this year, a rail car loaded with asphalt rolled away through the busy streets of Regina, Saskatchewan.

In 2014, CP Rail launched an investigation after two separate runaway incidents through the streets of Edmonton.

The use of remote control belt packs to assemble trains is widespread and growing. Safety officials have been paying much closer attention to the practice, following a spate of derailments and crashes stemming from incidents where ground crews have lost control of the rail cars.

​