Cars will be around for our entire lifetimes. I do think that having a modern rail system in place would be great, to complement cars. In cities it's easier and makes more sense, but there will never be a train that comes to my house, and if there is, I'm moving, because I sure as shit don't want to live next to a train. I'd love to be able to jump on a cross Continental, high speed rail to go on affordable trips, but that isn't possible and won't be. As long as I can pay roughly the same for a flight to my destination, and get there significantly faster, I will probably never opt for a slower option, and I'm definitely not in the minority by saying that.

With that said, assuming we spend the next 50 years eliminating cars and moving to rails, it still won't touch emissions, because cars are not the leader and are continuing to get better and more efficient. So starting now would be convenient and not a bad idea, but it won't change anything substantial from an emissions standpoint.