"We had three experienced people at the front of that train," said Tom Griffith, lead investigator with the Transportation Safety Board.
"We need to understand the environment in which the crew was operating and why they made the decisions they did."
Train 92 was travelling at 67 mph — almost 108 kilometres — when it derailed west of Toronto on Sunday, killing the three engineers and injuring 45 passengers. The speed limit while changing tracks at that particular switch, just east of Aldershot station in Burlington, is 15 mph — or 24 kilometres.
"Why were they doing that? That's what we have to find out. That's the hardest part of this investigation," Griffith said.
"No crew sets out to have an accident. Sadly, this crew paid the ultimate price, they lost their lives."
Two of the engineers, Peter Snarr, 52, and Ken Simmonds, 56, both of Toronto, each had more than 30 years in the industry. A trainee, Patrick Robinson, 40, of Cornwall, Ont., joined them in the cab to observe. Although Robinson was new to passenger trains, he had 20 years experience with freight.
Investigators haven't ruled out the possibility that Robinson was driving at some point, under strict supervision from the more senior drivers.
The train's black box is key in the probe — it records the train's speed, brake pressure, when the brakes were applied, and whether the whistle was blowing. However it does not record voice, which would allow investigators to hear the engineer calling the signals.
"The person at the control would call it first and be answered by the second locomotive engineer, so that would tell us from the sound of the voice who was actually operating that locomotive," Griffith said.
The train's locomotive and one passenger car flipped onto their sides and crashed into a small building next to the tracks, while another passenger car was left leaning precariously in the aftermath. Three other cars were vacant.
The switch the train hit is one that is "very rarely ever used," and requires the train to slow to 15 mph instead of the usual 45 mph.
"Because of other train movements in the area, they had to use this switch, which is still legal, but is a much slower switch than most crossovers," Griffith said.
Still, he added, the signal indication would have been yellow, telling the engineers that they had to reduce the speed to 15 mph.
Two people with 30 years of experience shouldn't have been travelling at that speed, at that location, at that time, Griffith said. They should have gotten the signal about 2.5 kilometres, before the switch, early enough "that if they wanted to slow down, they could have slowed down."
The train's black box recorder shows the brakes were not applied before the crash, and investigators haven't found any indication that the signals were malfunctioning.
CN, which owns the tracks and leases them to Via, said Thursday their investigation indicates the rail traffic control centre and signal systems were functioning properly at the time of the incident.
The accident has left experts and observers struggling to understand why the train would have hit the switch at such a speed.
Ray Marchand, general manager of the Canadian Safety Council, said the normal route at that particular area of the busy corridor would have been straight through, and even though the engineers were advised they'd be switching, they may have missed the signal.
"They just hit the switch like they were going straight. It could have been distraction, complacency, human error," said Marchand.
"This is not just a little bit over the speed. At that speed, they weren't going to stay on the track. They (must have) thought that they were going straight, like they probably did a hundred times before."
Rail remains the safest mode of transportation, experts stress, but they say accidents can always happen when people are involved.
In the U.S., Congress has mandated an additional safety measure called positive train control, a computer system that can warn, and if needed, override the conductor. It's currently installed in some Amtrak trains.
"Everybody thinks he or she is a good driver, but the average driver in an hour makes easily five or six mistakes," said Doug Bowen, managing editor of RailwayAge, a well-respected trade publication.
"In the case of trains, the vehicles you're playing with are usually a lot bigger, and so have much more visible and damaging results."
Via spokesman Malcolm Andrews said the company welcomed the findings, but cautioned against drawing any conclusions or laying any blame before the formal investigation concluded.
"It's far too early in this investigation to jump to any conclusions whatsoever," said Andrews.
"There's an enormous amount of work that's yet to be done in terms analysing exactly what happened. Let's allow this investigation to continue its course."
Via CEO Marc Laliberte also posted a message on the company's website, asking people to honour the three lost engineers.
Passenger injuries in Sunday's crash ranged from minor to a broken leg, a back injury and a heart attack.
At least two class-action lawsuits have been launched.
Investigators haven't yet looked at the mechanics of the locomotive, or parts such as the throttle, to ensure it was working properly, but note there was so much damage at the front of the train that it may prove difficult to examine.
Both the TSB and the New Democrats urged Transport Canada to install voice-recorder boxes on all Canadian trains.
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