There is a very handy booklet available
KAY, P A Guide to Railway Research and Sources for Local Railway History
Teignmouth, Devon, SSG Publications. 1990
A quick google located a copy for sale for ??5 at the following link
http://www.antiqbook.co.uk/boox/rec/45365.shtml
Although this is pre internet information explosion it has many useful tips and guidance for those interested in getting in to 'proper' research.
Regards
Mike