I think the example of the Bristol to Bath Railway Path through the likes of Fishponds, Staple Hill and Mangotsfield tells you all you need to know about the likelihood of Heavy or Light Rail ever happening close in to the centre of Bristol once the potential route in question becomes a cycle path.
But are all rail options on the Bristol to Bath railway path really gone?
The Avon Valley Railway still have aspirations to extend close to Bath. Furthermore, could not the lack of any immediate prospect of commuter rail reintroduction on parts of the route be more due to lack of political commitment than resistance from elements of the cycle lobby?
Could there not be room for both?
You'll note I very carefully worded my post to restrict what I was saying to close in to the centre of Bristol ie Fishponds, Staple Hill and Mangotsfield. I am well aware that the Avon Valley Railway have proposals to extend to a "Bath Riverside" station, which is feasible, and can build on the arrangements already in place to share the route with the cyclepath.
The cycle lobby were vociferously opposed to the prospect of Bus Rapid Transit alongside the cycle path through Fishponds, Staple Hill and Mangotsfield a decade or so ago, which was probably fair enough, given how indescribably terrible and utterly flawed that proposal was. However, that experience scared the politicians and transport planners so much, that they have never dared to try and propose the cycle path for any other use - even what some of us would perhaps consider more benign light rail purposes - since, and nor do I personally think they are likely to any time soon.
Leaving the above aside, in terms of whether it is actually possible in terms of room for both between Central Bristol, Fishponds, Staple Hill and Mangotsfield, the route does seem awfully narrow in places, but with modern technology, and a number of schemes with bigger obstacles to climb making the Restoring Your Railway cut, who really knows?