Yup, ceramic rail insulator. I have one as a garden ornament: it came from London's Metropolitan line and was made by Doulton. And indeed, very similar ones are still used on the Tube, so we in London see them all the time. Lovely article here: https://thebeautyoftransport.wordpr...of-transport-beauty-third-rail-insulator-pot/ The Tube is four rail working rather than three.
Wow! Well, now I know. I hope some day, somewhere, SOMEBODY asks me to identify one. Think how smart I'll look.