I liked this bit in Establishment Journalists Pride Themselves on Staying on the Official Rails: "Rails, after all, are meant to keep a vehicle on a predetermined track. It’s not much of compliment to compare a journalist to a smoothly operating train, always showing up at the official stations."
I'll take that point further: When people say you're derailing, ask where they want to send you. And remember that to someone who thinks you're derailing, you're not in a conversation; you're in a lecture that allows some comments from the audience.
Derailing is rightly a scary metaphor. For people who thought they were in for a cozy ride, dealing with the unexpected is annoying or terrifying. But many passengers in history have been trapped on trains that they would have gladly derailed.
Shorter version: Derailing is just a way to say someone is thinking freely. Authorities hate that.