Originally uploaded by ridgway
666 is the Number of the Beast. The reference is from the Book of Revelations.
According to the Wikipedia there are all sorts of explanations and some authorities say it was 616 anyway. I had not come across this weird number stuff until recently. All I knew about 666 is that it was the number of my local trolleybus. When London trams were replaced by trolleybuses in the 1930s, they added 500 or 600 to the number of the previous tram route. 666 replaced tram 66 in 1936 and went from Edgware to Hammersmith via Cricklewood, Willesden and Harlesden. I never heard it referred to as the Route of the Beast, which suggests that this nonsense could be a recent invention.
To bring things up to date, the 666 trolleybus was replaced by bus route 266 in 1962 (above). The first replacement buses were Routemasters, which ran until the route was converted to one person operation. So you could say that 266 is the current number of the beast. Only now it goes to Brent Cross shopping centre instead of Edgware.