Typically fuel soaked items don't burn - the heat released from the vapor flame above simply causes the liquid to vaporize, absorbing the heat and limiting the temperature to the boiling point of the liquid. See candle wicks and kerosene lamp wicks for examples. While those do get consumed it's the part of the wick that cannot maintain a flow of fluid that gets burned.
The tracks themselves aren't protected by being soaked in fuel and they would have heated to the flame temperature, both expanding and buckling lengthwise from the heat, but also lowering their strength where the weight of the tankers caused them to buckle.
I'd guess that the underlying structure is fine and that they may have to replace the sleepers (ties) along with the rails, as the concrete ties would not have been effective at storing fuel and gotten over-baked. Gravel and dust aren't great insulators, but the energy required to change the phase of liquid to gas is considerable and puts a limit on how hot the liquid can be.