In the days when Linwood was still a pipe dream and 153 came out of Pot Yard and 173, 453, and 155 originated at Spencer, all usually ran radio. As Tim says, when tonnage was light the trains would have all the power on the head end. But what most don't know is that we would often run them radio even though all the power was on the head end. This to make the Charlotte pickup much easier.
Leaving Spencer the radio equipment would be activated and there would be no MU cable and no MU hoses coupled between the head end power and what would become the mid-train power at Charlotte. The engineer knew to keep his radio unit isolated and its feed valve cut out until he made his pickup at Charlotte and while he was making his brake test at Spencer.
On arrival at Charlotte, you cut behind the head end power and leave the train and the radio car/mid-train locomotive on the main, go get the pickup and come back to the train. Didn't have to radio the train because it was already done.
Alas, this practice was frowned upon when it was discovered. A memo would come out about once a year prohibiting this and we had to lay low for a while. But no one ever came up with a good reason why this was a bad practice so we were right back at it again until the next memo came out. Good times and good railroading and lots of fun.