“So why can’t train tracks or any highway be a barrier to separate…how do you define a barrier? is there a shiur how wide etc..?”
I have no idea. I really don’t know what defines a barrier; I just do what the posek hador says.
“How come no rebbe makes an official statement that the Flatbush eruv is kosher? There was a kol kore written by many gedolim, not too long ago,that it’s not a good eruv?”
I’m not sure how much of this thread you’ve read over, but it’s pretty well-known that Reb Moshe ruled it assur. If you want to be a respected Rabbi in the frum community, you probably don’t want to go arguing on Reb Moshe without really good reason.