I've have twice been through the process of tearing down, rebuilding and selling the 313, and now I'm working on the third. I'd be glad to share what I've learned.
The information on the Lionel instruction sheet and Greenberg's Repair Manual don't come close to having the needed information. I found what I needed in "Toy Train Repair Made Easy" by Ray L Plumber. I had the good fortune to stumble on an excerpt from the book on Amazon or Google that just happened to be about the bridge so I didn't have to buy the book. The lowest price on Amazon now for a used copy is $89.95. A copy sold on Ebay for $12.50 on April 6, but that seems like a fluke.
I had two of the bridges from a collection that I was selliong on commission. I completely disassembled both. The means removing the the towers, gearbox, contacts, etc. Then I rebuilt one using the best of each kind of part and got a very good price. Then I assembled a second bridge and got a lower price because it didn't look as good. The third I'm going to repaint (very rusty) and keep for my own collection.
Making the bridge work properly isn't easy. I found the most difficult part to be adjusting the contacts for automatic operation and train stopping. Reassembling is easier if you have three hands with small fingers.
I be glad to talk with you and help as much as I can from my experience. Send me a private email - mlaughlinnyc@yahoo.com - and we can trade phone numbers and talk. I can also send you the critical info from the book.
I didn't know about that 22 page TCA item that was mentioned. But for me now, that's water already over the dam.
Malcolm Laughlin