You certainly can build trains wherever you want, but it comes at a cost that’s not necessarily worth paying everywhere, as it comes with both short term and maintenance costs. I say this as someone who works in rail and is passionate about it; in some locations there isn’t the demand to run the kind of high frequency service necessary to remove the need for car ownership. You can be better off with a demand responsive bus service, for example, to connect to your long-distance, high speed links.