TfL have announced official names and colours for the Overground lines:
We’re giving the lines on the London Overground names that celebrate London’s wonderful and varied cultural heritage. In this blog post, we tell you all about the names, the stories behind them and when the changes will come into effect.
Good point. I wonder if it’s because there’s not an obvious abbreviation. “The Metty”?
It’s always been the Met line?