Looks like a covered utility trench, ironically with the cover off.

So a trench.

Turtles can’t derail a train. Turtles go squish. Even if you piled them up 6 foot high in front of the train.

This conjures an image of a very determined Bert the Turtle smoking a cigarette and declaring “I’m gonna derail that fucking train.”

Yeah “to stop them being blocked by the rail and giving up and going another direction” sounds much more accurate.