I have



and I'm not the only one

 

Plus RAF Ballykelly in Northern Ireland when it was still an active airfield.

 
Road/rail sharing is a reasonably common thing in Switzerland, though I can't think of an example where it includes a single-carriageway bridge.

If you want a really extreme example of sharing, Google Gisbourne Airport where the railway line crosses the runway.

 

And that's shared with a road crossing too - but that's a transporter so at low level.

 

The Avonmouth M5 bridge has a 30m clearance over the river (not sure at what tide state) so you would need about 1km ramps on each side at 1 in 30, or 2 km for 1 in 60.   The Prince of Wales bridge over the Severn has a 37m clearance, the old Severn crossing 47m. 

For comparison ...

The Rendsburg High Bridge over the Kiel Canal has a 42 metre clearance, and a spectacular spiral on the north side for trains to loose / gain height. "The required maximum incline of 1:150 (a one-metre gain in altitude for every 150 metres in distance) made it necessary to build elevated embankments and access bridges of about 5.5 km (3.4 mi) at each side" says WikiPedia

 
No idea how to quote on here - but to respond to post 3:
How far away from the river crossing would the junction have had to have been to gain/lose enough height to reach the navigational headroom under the Avonmouth bridge (my guess is several miles)
A9 - do you mean the Dornoch Bridge - yes definitely a political “no” to a combined road /rail crossing there - though it avoided the possible can of worms of what to do with the bypassed bit.

 

You ask a lot of very good questions - and indeed I am seeing so much that is different in Sweden - not necessarily all 100% positive, and it's huge food for thought.

I have a train to catch ... will be back with further comment on the bridge and so much more.

 
Not wanting to go go the political road,so I am not going to Check what party was in power at the time.

1)Portishead line when the M5 over the river Avon was first mooted,COULD a rail bridge not have been constructed along side?

Those who live in Pill short distance to the railway line which could have crossed the river dropping down to the now park and ride

2)Similar to the second Severn rail crossing could it not have a had a rail line along side?

3)If I recall there was a plan to build a bridge across the A9 road towards Perth,
when plans was agreed for a road/rail bridge the government stepped in and said a road only bridge
So the trains still have to go around the estuary to get to the other side.

4)In my opinion if the party of the day,the Channel Tunnel would have been a bridge for vehicles only,
thankfully the French were stronger in what was best.
We all know who didn't like the railways and very rarely traveled on trains unlike our Queen Elizabeth the second   

 
Also on the Ribble Steam Railway in Preston. Until recently, oil tank wagons from Network Rail ran over the swing bridge.

[Geograph - Creative Commons Licence]

 
Try doing that on the Royal Albert Bridge. 

 
An example of a very rare practise from Sweden - I believe it was done at Connel Ferry in the past, and there was something similar at the avalanche shelter on the Kyle one - but it requires considerable precautions, is slow and only words in the most extreme of cases

The train driver has got out, checked the bridge, and pushed the button to lower the barriers ...



... before the train crosses the bridge.



It then stops on the other side to let the passengers off to have a look ... 





... and see the road traffic that was held coming over.



And then the passenger go onto the bridge to take a look themselves ...
... with the train manager taking souvenir pictures for the tourists