All in all a decent start. Despite the technical mistakes it DOES make you want to read more.
But the writer does leave some flaws. First, the bad guy sets up his lawn chair 20 feet from the broken rail. Even with the forward motion of the train, it's a reasonable bet he'd get wiped out himself as the derailment progressed.
And he tells of the conductor's face splatting the windshield hard enough to break the FRA certified glazing, just from hitting the broken rail. No way - at that point they'd feel a bump but nothing severe enough to break a window with his puss.
And third, even at 70MPH it does not seem all 50 autoracks would go over the bridge. As they derailed, there would be an accumulation of derailed cars that would act together with the brakes going into emergency. Yes, a lot of them might go over but I'm betting not all of them.
Even with those items [and there may be more I passed over] it's still a gripping tale. I hope no would-be terrorists are reading it as a text-book!