In message , at 08:45:52 on Tue, 26 Apr
2011, d remarked:
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.

That's built to carry three lanes of 40 ton HGVs.

Rail bridges OTOH seem to be somewhat slender in comparison.

The busway bridge is pretty slender too. Here's someone's picture of it
under construction.

http://mw2.google.com/mw-panoramio/p...m/12999233.jpg

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.

Buses and busways are narrower than trains and their tracks. The only
wild card is whether you have a pathway beside them.

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 ,

The route from St Ives to Huntingdon has only ever been speculation,
especially over the route it might take (the old trackbed's not
available for almost the entire length). Every suggestion I've seen
results in joining the ECML from the north, and the folks keenest on
reopening the line finish their route at "Huntingdon East" conveniently
not specifying the final mile.

coupled with the fact that it would have provided a useful diversion
route for the ECML.

Single track and non-electrified (ignoring the reverse at Huntingdon for
a moment) does not make a very useful diversion.
--
Roland Perry