The fact that planning permission has been granted suggests there are no proposals or policies to retain the possibility of reopening the railway in future. Double check this with your council and seek advice from Network Rail. Get assurances in writing. Look for evidence of past campaigns to re-open the line and don’t go ahead unless you’re absolutely certain there’s no chance of this happening. Has the line been built over elsewhere – if so, and especially if there’s significant building, this should give you some comfort. The deeds of the property will reveal any restrictive covenants or rights of way, so your solicitor must check these thoroughly, but would do so in any case. Finally, check whether any investigations into pollution/contamination have been carried out and if not check with the council whether contamination is likely.