To speak on my own patch, there's a fair bit one could look into on the heritage railways of North-West Wales, with scope for comparisons between them:
Tom Rolt describes how the Talyllyn Railway was still being operated to serve the needs of the local population when preserved in the post-war era (Railway Adventure). Similarly, I understand that the Ffestiniog/WHR, in addition to originally transporting quarrymen to work, have operated some morning commuter services more recently.
The rebuilding of the WHR through Snowdonia National Park required several special arrangements, particularly when reopened as far as Beddgelert, to avoid hordes driving there to board at the (temporary terminus). Plenty of scope for analysing environmental studies, etc here.
Historical comparisons could be made with the Rheidiol railway, which remained state-owned into the 80s(?): A BR steam-operated narrow gauge railway.
There's also the Snowdon Mountain Railway, which has apparently never regarded itself as a heritage railway, but rather a tourist operation to get the punters up the mountain. In this regard, its use of coal, oil-fired and diesel traction over the years could be interesting.
Of these, I'd guess the WHR has the greatest scope for a dissertation, but it's your project, of course.