Listened to BBC Three Listeners radio today and they interviewed a rail transport expert on the news of the airport wanting to create a new rail-link...
You can listen to this on the below link and fast forward to 2hrs 22mins 50secs.
http://www.bbc.co.uk/programmes/p03766n9#play
Some very interesting points and he clearly points out the most likely option would be a tram or light rail system similar to the Docklands Light Rail or People Mover at Gatwick connecting both terminals.
He did however make a very good point that regardless of what this rail link is, it still gives the airport a severe disadvantage compared to the other airports that have direct rail connections to the main terminals, and how passengers still need to use a seperate mode of transport to get up the hill, once getting off the train.
Anyway have a listen!