You are correct with higher voltages being easier to transmit over longer distances: that is the reason for "high tension" lines running across the landscape.

As for the throttle on a model railroad you are adjusting the voltage; the amperage doesn't change. In fact some of the better packs, until the advent of transistorized power packs, and most of the tinplate packs (Lionel, Marx, American Flyer) actually use variable transformers for voltage regulation.