So why would engineer honk at a curve?
He wouldn't.
A witness living in area reported a series of sharp train horn blasts followed immediately by explosions.
TAC Air (police helicopter) reported a CSX engine with lights on, hooked up to 18 cars.
There is no guarantee that the Amtrak engineer was the one who sounded the horn; that could have been the CSX horn, giving an audio signal--as well as providing a reference light--to a saboteur.
If you look at where the train derailed, a derailer could have been placed there specifically to send Amtrak flying into the pack of oil trains, with the saboteur hoping to blow car #2 (first passenger car) up and set the whole county on fire.
But he pushed the button a hair too soon, and melted #2 to the grassy area. Had he waited even one second more, it's likely many or all of those oil tankers (and lord knows what else was there) would have blown.
I think the train horn is key--the fact, especially, that it was not laid into like you'd expect in a real imminent collision, but that it gave "a series of short, sharp blasts" followed immediately by the explosions.
Remember it has already been reported that the engineer slammed on the e-brake...