Pretty sure there's enough space to have a tram-like train running along Speke Hall Road, to link up with the mainline either from the Jag goods line, or from Hunts Cross.
I think the idea has been mooted a few times in the Echo, but naught has yet to come from it. As John says though, something like that would only really benefit local users; what's needed is a dedicated rail link but as I said before, just extending a branch off the Northern Line from Liverpool South Parkway would likely be considered prohibitively expensive under the current economic conditions.
John, when I talk about the short termism of small minded men and women it's largely aimed at a council that lacks ambition; but it could also be aimed at Merseytravel and Merseyrail, whose ambition in extending the network has seemingly been limited to electrifying existing lines rather than build new ones. (Admittedly I don't know the funding details of the Roby/Huyton stations expansion but I'm guessing this did not come directly out of Merseytravel's existing budget but was specifically earmarked for this purpose.)
Yes, I recognise that I am probably being overly harsh and yes these are difficult economic times. I could go on but it just seems that an engineering project on the scale of the loop line, which arguably took place during similarly difficult economic circumstances, would be considered pie in the sky today. And don't forget the Merseytram debacle.