Can I use the old 4.5V and 12V blue/gray era rails with newer trains?
You can absolutely run RC and PF trains on 12V track. You can leave out the center conducting rail since it won't be delivering any power to the train, unless you are planning to run both 12V and remote trains.

The main LEGO train sets have all used the same rail gauge, which is the main thing that matters.

If you'd like to use both your 12V track and the new RC track that you get, they can be joined together in various ways. Here's an example from a LUGNET thread:



There are also a few places where you can get third party adapters if you'd prefer to go that route.