George Schreyers Large Scale Trains an excellent resource with great technical tips and a gold mine of information about various different brands of trains. The site has been on the web for many years but does not seem to have been updated in a long time. I hope George is okay!
Family Garden Trains. This site has everything from soup to nuts for the beginner or veteran of Garden Trains. This is where we got the information to build a trestle and also the instructions for a cool method of elevating a garden railroad. There is a whole bunch of other stuff too.
Plastic Gear Vendor. I stumbled on this site searching the web for a plastic gear to repair one of my Bachmann locomotives. If you can take the time to carefully measure your gear, I believe they have most any size or configuration you could want. They provide extensive search capability to find just the right gear. They have a whole bunch of other stuff too.
Electrical Components Vendor All kinds of wiring, terminations, heat shrink tubing and most any electrical item you might need. They have so many parts and supplies it might take you a while to sort through and find which one you need.