It's an accident of history, as it were, that the bridge is four lanes. I remember as a nipper them widening the bridge - originally it was just the two lanes on the station side of that old metal barrier, but obviously it couldnt take the increasing amount and weight of traffic so they built on the other side. I assume it's a weight/structural thing that it is so wide, not because anyone envisaged four lanes of traffic going over it at that point.

I have a photo of the old bridge which my Dad took I think which I'll post, out of interest, if I can find it!