I have attached some documents for my preferred railroad, but they might not be useful to you. I also have some from a company, Custom Signals, that I don't believe is still in business, but they still provide some useful info. However, they are copyrighted so I can't post them here. Contact me via e-mail in profile and we can discuss.
Of course, if you are planning to prototypically model a particular railroad, your best source might be from publications about that railroad or from that railroad's historical society. Finally, search on line for "The Signal Engineer" and "Railway Signaling Magazine;" both may have useful information.
Chuck