It wouldn't be a new line since the Phillips Park line already exists, it would be a new service on an existing line for when the existing infrastructure of other methods of public transport exceeds capacity. It's already is doing that more frequently than it ought to. There's co-op live, hotels and business space as part of the City expansion on the way, as well as the Euros coming up. Though, come to think of it, I suppose they will make sure nothing will clash with that and avoid embaressing the country as best they can.
What would probably be needed is a station, a bit of track to offload the trains onto at either end, possibly a bit more(access and the likes). It was just an idea floated by a few people, rather than an expectation(nobody is surprised to hear it's deemed too expensive to justify... for Manchester). How it would be done and how much this would cost are things that I don't think anyone should be pretending they have much knowledge about unless they genuinely do.
I'd wager that it would cost a lot less than the ridiculous waste of public money that the HS2 is going to do, which benefits mainly London, who already have it better than most of the UK in terms of transport infrastructure. If the Eastlands area continues to attract more property investment and business investment though, it's only going to get more expensive down the road. If at some point, they have to accept that there's only so far adding more buses and trams onto the same routes(roads and tram lines which already have congestion issues) will get you.
