Reddit reviews North American Railroads: The Illustrated Encyclopedia

While it's not the most scholastic source (the owner sometimes writes subjective things on objective topics), AmericanRails is probably the most comprehensive easy-to-navigate free site and is a great way to learn about general North American rail history for free. Wikipedia is hit-or-miss as always, with some very comprehensive articles and some that are just a few lines and a picture, but the fact that it's free makes it worth a look.

You mention the east coast, do you mean the Northeast Corridor specifically or the entire coast? If you mean the NEC, the line was operated for most of its history by the New Haven RR and the Pennsylvania RR, which would later both become Penn Central before Amtrak took over operations of the NEC. If you mean the entire coast, I'll need another comment for that.

Unfortunately this hobby's best information about historic railroading is almost entirely book-based, so to get everything you'll have to invest in some books. Anything by Kalmbach will be accurate, comprehensive, and hopefully not too hard to read, this is a good historical summary. I have this book, and I think it's a good summary; the language isn't difficult and there's plenty of pictures. I've seen people recommend this one, it's less of a history book and more of an encyclopedia of nearly every major North American railroad. These books cover general topics; there's been scores of books written about every railroad that exists/existed. If you can help refine the location/area, we can try to find more specific information.