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I thought I recognised that. There are no railings to either side of that gate and there used to be an installation of sensors and speakers there that made the noise of tinging a stick on the "ghost" railings. Apparently it was significant since the railings were some of the many that got chopped off during the second world war to be made into stuff (but apparently all just got dumped in the Thames as there wasn't actually any call for low-grade iron).
Old: Malet Street Gardens, WC1