I think the advantage of tram-trains for airport services is that they only need a reasonably cheap new branchline at one end, so they seem the second best option after a proper railway branch. They make particular sense when a tram or stopping train from the airport to the centre doesn't look that likely. On the other hand, if Bristol ever gets a heavy rail metro system, that would also be good. Can a Clifton Underground Funicular revival be attached to any of these schemes? It was four-track I believe, which is surely one of the most unusual four-track railway sections in the country...