[Maple Leaf]

Here's the damage I was talking about:


The gravel(?) between the cement ties is missing.  I'd think this means that something under the gravel burned through to some void.   This would allow the burning fuel to get down to the span's main structural component.  I presume that is some sort of steel structure.  The damage to that might not be as obvious as the deformed rail, but the heat could alter the strength of the steel. (Fire effects on steel;  fire effects on concrete)

Steel Reinforcement in concrete known as "re-bar" will crystalized when exposed to heat (like welding) and become brittle.  There is a special grade of re-bar specifically designed for welding. The heat generated by fuel in the rail cars could have very well reach temperatures that could spald the concrete and thereby lessen the bond between the concrete and the re-bar.

[Maple Leaf]

Here's the damage I was talking about:


The gravel(?) between the cement ties is missing.  I'd think this means that something under the gravel burned through to some void.   This would allow the burning fuel to get down to the span's main structural component.  I presume that is some sort of steel structure.  The damage to that might not be as obvious as the deformed rail, but the heat could alter the strength of the steel. (Fire effects on steel;  fire effects on concrete)

Steel Reinforcement in concrete known as "re-bar" will crystalized when exposed to heat (like welding) and become brittle.  There is a special grade of re-bar specifically designed for welding. The heat generated by fuel in the rail cars could have very well reach temperatures that could spald the concrete and thereby lessen the bond between the concrete and the re-bar.