Anybody interested in the above topic might like to take a look at the book of the same title. It's published by the Institute of Railway Signal Engineers so it's a pretty authorative source. I've just got my copy this morning so I have yet to read it apart from the odd paragraph. But from those paragraphs it is very accessible to even complete beginners.
I don't normally recommend books but this important subject has sadly in the past been neglected by the general media; a hole hopefully filled by this book.