Some interesting comments here. Eightf is spot on about tram train suitability for Bristol: Portishead-Centre-Avonmouth/Filton etc. They could solve the problem of Temple Mead's peripheral location quite cheaply, by building a tram line through the harbour area to join the Portishead branch near Ashton Gate. This could eventually be used by tram trains on routes such as Weston-Yate or Weston-Bath-Wiltshire.

Mention of Germany and diesel engines on the roof reminds me that I travelled on the BOB railway from Munich to the Alp foothills (Tegernsee). It uses new tram-like dmus with roof-mounted engines which means they're quieter and less vibration-prone than underfloor engine units. They're the nicest dmus I've been on, and show you don't need electrification to get quiet trains. I wonder if roof-mounted engines would work with the UK^▸ (United Kingdom) loading gauge, though.

There are pictures on the BOB website (Bayerische Oberland Bahn). Incidentally, BOB is a Veolia (Connex as was) company.