On Mon, 25 Apr 2011 17:53:28 +0100
Roland Perry wrote:
Most of the weight a bridge has to support is its own weight. When you
get into those sorts of tonnages the weight of the vehicle crossing it
becomes only a small percentage of the total weight so the overall structure
of a busway bridge I suspect is not much less than that of a railway bridge.

Let's say 90% of the weight of the bridge is required to keep itself up, and
only 10% is represented by the safe load above it. That would indicate that
a bridge for a 14 ton bus would need to weigh 140 tons. Are you suggesting
you could run a 150 ton train across such a bridge, rather than needing a
1500 ton construction?

I don't know. What I do know is that modern road bridges and viaducts to me
seem to be very over engineered given the total weight they'd ever be expected
to carry. Eg , that M1 viaduct that had a fire underneath. Rail bridges OTOH
seem to be somewhat slender in comparison. So while I may have phrased it
wrongly I still don't think a replacement rail bridge would have been much more
hefty than a busway bridge.

A bus is narrower than a train, you can't get away from that basic fact. And

Not by much in this country. Buses are what, 2.5 metres wide? The UK loading
gauge is 2.8 max.

There isn't a direct road between the villages which the busway connects, so
it would be very hard to run a bus in the absence of the busway. The bigger
problem is that those villages won't create enough custom to fill a bus
every 20 minutes, let alone a train.

True, but a rail link from huntingdon to cambridge via ST Ives may well have
done , coupled with the fact that it would have provided a useful diversion
route for the ECML.

B2003