City and South London railway

I'm reading a book called "Underground Overground" which looks at the history of the Tube. I read about the HQ of the City and South London Railway being built at Moorgate and about the company's arms. Then I realised that I'd noticed the carving before and that I walk past it on my way to work, so it was an obvious blip

The carving is meant to represent a bridge over water, the two arches of the bridge also resembling tube tunnels. It's supposed to show what the railway did: link the City with South London.