I'm going to be a bit pedantic Kentblade .... but doesn't that photograph actually show the congestion caused by double-decker buses, not trams?
I formed the opinion that its buses that really clogged up the traffic in London (and less so elsewhere), they are just too big to get round corners quickly enough in the correct lanes as they swing out and round they block traffic both ways. It is fashionable to blame cars but being a cyclist, I know first hand the filth these things emit and I also note that on many routes up here, they are run virtually empty throughout the day and could be replaced by big cars.
But no one is prepared to call a spade a spade.