Lines converted up to now are all in metropolitan areas. Future expansion will probably include tram-trains Manchester - Rose Hill and Sheffield - Doncaster, although the latter looks a long way off. Proposals to convert Watford - St. Albans and the Isle of Wight line were not pursued; one of the stumbling blocks in both cases seems to have been the likely loss of through-ticketing to and from the national network. Assuming this issue can be resolved, which other lines might be considered for conversion?