Books are a good source of information although they tend not to be very technical, except for those on electric traction.

Books are of course very good for historical information and photographs.

Papers published by professional institutions such as the IEE and the IMechE are an excellent source of technical information.

Copies of papers can be obtained from their respective libraries.

Magazines and journals are another superb source of information although articles on railway electrification can be sporadic.

I recommend 'Modern Railways' as being among the best railway magazines in the UK.

Conference papers are good source of information, but do have the tendancy to be marketing exercises for the company of the speaker.

Nonetheless, they are a good way with keeping up to date with the latest thinking.

Technical standards are probably the best source of technical information, but you need to be aware that they might not always represent current 'best practice' due to the time that it takes for them to get updated.