I'm not sure I get the point about the class of the track on the trestle. Are you saying that not all of the damaged materials need be replaced while still complying with FRA rules.

It always seemed to me that with class 1 track standards, the question is not IF you might derail, but only WHEN. -- You WILL be biting ballast for lunch.

I always thought It was considered safe at 10 mph, only because the consequence of coming off the track at such low speed was usually relatively minor. Other than scaring the horses, it was mostly just a common annoyance.

But falling off a trestle doesn't seem so minor! -- Therefore, even if the rest of the track is only class 1, track on trestles and high embankments should be really solid - say maintained to at least class 4.