It was a scene many won't forget -- a collision of trains on the east bank of the Fraser River across from Paddlewheel Park that sparked an explosion and a massive fire that sent thick, black smoke high into the sky.

In a report released this week, the Transportation Safety Board provided perhaps the most comprehensive account of what happened on that day, Aug. 4, 2007, when a collision of CN Rail trains led to a massive fire and spilled thousands of litres of fuel.

Here's a look at what investigators discovered:

Two CN Rail managers -- the area superintendent and the senior engineering manager -- were assigned to the south yard (the old B.C. Rail site) to switch cars due to a shortage of staff that day, a Saturday.

The area superintendent operated a remote control system use to control the locomotives and the engineer manager's main functions were those of a yard helper -- to line switches, couple and uncouple cars and review switch lists.

On that note, investigators raised two concerns:

Although the area superintendent had formal training and was qualified to use the remote control system, "his practical experience switching cars at this location was limited to two previous occasions for short periods of time and with fewer cars."

Both employees had participated in trial runs when CN made a change to handling longer and heavier lengths of cars, but neither had actually used the system to control the speed and braking on the descending grade in the area where the incident occurred.

Both managers involved had worked about 60 hours over the previous five days, had not had a full day off in over two weeks and worked in their supervisory capacities the day before.

Investigators qualified that finding by noting they were able to report to the yard at 7 a.m. the following morning "rested" and had obtain sufficient rest over the previous 24-hour period to meet company requirements. However, time worked over the previous seven days was not recorded and "could not be accurately validated," investigators noted.

The two were working at the north end of the south yard and after waiting for another train to finish switching, they went to uncouple 53 cars from a 95-car length of loaded cars. With the help of two locomotives and a "slug unit," which provides additional braking -- it was moved northward and the "cut" was made behind the 53rd car.

The 53-car length then had to be moved about 12 cars further north along the "pub track" -- the closest to the river of two tracks along the east bank -- to clear a "bull switch" and allow the operators to move the cars into another section of the yard.

At 10:13 a.m. the remote control operator put the unit's throttle into position four and accelerated the train to 1.98 miles per hour. One minute and 12 seconds later, the operator moved the throttle up to position seven to raise the speed to 5.71 mph.

Seven seconds later, the operator applied the locomotives' independent brakes -- the air braking is not normally charged on cars equipped with the system during switching movements -- lowered the throttle back down to position four and jumped off the train to apply the bull switch.
But the train kept moving faster and it was at that time that the engineer of an incoming train advised the operator they were preparing to enter the yard using the crossovers.

By the time the unit's throttle was placed in "stop" and the brakes fully engaged the speed rose to 7.46 mph. And when the unit was put into "emergency" 49 seconds after that, it was traveling at 8.66 mph.

The engineer of the inbound train stopped his train, jumped off and attempted to apply some handbrakes on the oncoming length but to no avail -- it collided with the 13th car from the head of the train at 9.33 mph, roughly five and a half minutes after the decoupling in the yard began.

Loaded with gas, the 13th car exploded as did one next to it also carrying fuel, creating a massive fireball followed by a blaze that lasted well into the evening. The scene drew a parade of onlookers to Paddlewheel Park and generated a fire so intense that water bombers were called in to drop fire retardant.

Investigators later determined the 53 loaded cars, weighing 7,000 tonnes in all, were simply too heavy for the braking system applied. They also noted that CN Rail had conducted a risk assessment the day before the collision to determine process for handling heavier and longer lengths of cars but was not put in place at the time of the incident.

Following the accident, a higher-level, more thorough risk assessment was conducted that produced a chart governing the number of loaded cars or tonnage that can be handled with a given amount of braking.

Had it been in place on the day of the collision, between four and five locomotives or slug units would have been used, with the length limited to 50 cars and the tonnage limit to 6,500 tonnes.

On the day after the incident, Transport Canada imposed conditions for CN activities along the stretch. They were lifted on Aug. 17 after CN issued the new protocol to employees.