Worth a scan at Smiths or Menzies for the mag with the most current running details, though you are getting near the end of the season (tends to follow school holidays)
Railway Heritage is probably the best for running length, stock, contact numbers etc though Railway magazine and Steam World (or is it Steam Railway?) cover the latest info. Heritage is probably the best for news and proposals and appeals, though all the mags mentioned are good. I suggest you follow the news articles as these are always on the look-out for rumours and schemes picked up by local knowledge and newspapers. (be careful of the fanciful, like Severn valley proposing Eastwards extension and North Yorks railway extending to Malton...if anything these are on a wish list a generation ahead.
However little is mentioned of the fledgling Helston railway, Cornwall (as part of Trevarno gardens, but on the old trackbed) and the Yorkshire Wolds proposals near Fimber, on the Driffield line. (OK I know you asked about Somerset and Dorset but not so many know about those two above, so there will be others!
An excellent map came free with Railway Heritage in 2008 care of an outfit called Trackmaps.co.uk which is well worth a look.
On said map are the Avon Valley railway, Bitton near Bristol...East Somerset railway, Cranmore and the Gartell Light railway near Templecombe...
Suggest you Google Somerset and Dorset and see some of the staggering ideas for reopening the whole route...and why not? Keep trying the Euro lottery!
Happy hunting. Jim