Worthing3104
Hello & Welcome to the Forum!

I am not quite certain of your "Garden Train" plans.  Are you speaking of running O Scale or Large Scale outdoors?
If you are considering Outdoor O, your track should be your first concern.  The normal tin track used for O scale is not adequate and will quickly become rusted junk. You will need to purchase stainless steel or brass track with UV resistant ties.
 Power? as much as you can afford to purchase.
Feeder lines? take a voltage tester, go around the track and look for voltage drop, that's where you'll need a feeder.

O Scale in the Garden is quite common in England/Great Britan, you might want to look up Brit sites to check out what done over the pond, if you plan on Outdoor O.

If you are going large scale, all the same applies...

Happy Rails!
dave