There's an instructive chart of the Lac-Megantic derailment at the National Post. Professor Bruce Dunwoody of the Faculty of Applied Science at the University of British Columbia has done some calculations, which the compilers of the chart use as annotations to the diagrams of the wreck. The math suggests that a train of freight cars taking the curve at 101 km/h (60 mph old style) might have been "teetering" rather than simply rolling to one side. Throw in some rocking and rolling affecting the trucks (bogies, if you're on the other side of the Atlantic) and facing point switches at the west end of the yard, and there might be a picked point involved in the derailment.
Next question: was this yard equipped with propane-fuelled switch heaters? There's still something about the force of the explosion that doesn't square with the lading of the cars. Our cars and lanterns use refined fuels for a reason.