Depends what you mean by "light rail".
I think there is a very strong case for moving to a light rail style of operation on rural branch lines which don't (or only do for operational convenience) run through onto the mainline for any considerable distance. This would allow things like the following:
- Low floor platforms with on-the-level foot crossings mean building a new station is much, much cheaper
- Drive on sight simplifies signalling considerably; if there is a passing loop a simple train staff and sprung points could be used, requiring no signalling staff at all
- DC electrification is possible at a much cheaper cost than mainline 25kV kit, and the equipment is typically quite visually attractive so fits well in scenic locations
- If operated locally like the Stourbridge Shuttle, which is effectively light rail, staff may be cheaper
There is the downside that you need a spare vehicle for the route rather than just getting one from a TOC's general pool, though.
Tram-train type vehicles could be used, allowing the odd bit of inter-working with "normal trains" at the main station end. The Sheffield vehicles look quite well-suited, with the standee/accessible bits at the outer ends and seated bit in the middle for longer trips.
There are a few lines this could work for -
- Kirkby (or later Skem) to Wigan
- Ormskirk to Preston (though you'd need batteries or 25kV capability for the WCML bit)
- A number of Manchester suburban branches, though just adding them to the present Metrolink is probably easier
- Were it not for EWR and freight, the Marston Vale
- St Albans Abbey
- Windermere if the through running was removed, you could potentially run it down the road to Bowness instead (Windermere itself is nice enough but isn't much of a destination in itself so you always need an onward bus or taxi)
- Blackpool South, and possibly a Fleetwood branch. Though these may be best as part of the existing tramway
I'm sure there are plenty of others.
Edit: someone mentioned the Thames Valley branches (including Windsor, where it could perhaps run on-street to Riverside and add a couple of stations along the way for Chalvey and Eton North?), and I'm going to take that and raise it with the Cornish branches too. Potentially even the Conwy Valley and Borderlands Lines - the former might for example allow the trackbed to be dropped down in sections to allow the river through which might help in it avoiding being washed away.