If you are out of supply, your movement is halved. OK, I get that.
But what if you are on the railroad and you are out of supply, is your movement reduced to 15 or do you move half your normal movement?
"6.24 Units do not have to be in supply to use the *full* Railroad Movement bonus, nor do they have to be within a leader's radius."
But of course, they must be using Rail Movement -- otherwise, if out of supply, the normal restrictions on movement apply, no matter what sort of hex you happen to be in.