You are misunderstanding the nature of the problem. I like trains but they can't and don't address the issue the OP is raising. Even if the US already had public trains it still isn't a "last mile" problem. Especially in the western US, it is a "last hundred miles" problem.

No public transport system that remotely makes any kind of economic sense, either in terms of infrastructure or operational cost, can replace the established network topology that exists for cars in the US. The connectivity is much more like a mesh than a hub-and-spoke model. Even though the US has a strong regional jet system that connects arbitrary nodes in that graph it still doesn't entirely avoid the "last hundred miles" problem.

A lot of American long-distance travel is not between two big cities. Even in Europe, similar kinds of routes have no train service and limited bus service.