So the previous owners tried to kill the dog by tying it to the tracks? Seems the sign of an increasingly sociopathic mindset. If they decided they could not handle the dog anymore, could they not have given it up for adoption or at the very least given it a sort of injection so it could die peacefully as opposed to being hit by a train while tied up?
Unfortunately there are a lot of cruel people out there.
I personally think that those who are cruel to animals are unfit humans.
Fortunately there are also a lot of caring, decent ones.
Might not have been the previous owner, might just have been some butthole(s) doing it for kicks.