Welcome aboard! I don't have a garden railroad since running one in the Alabama summer heat isn't my idea of fun.
I had several friends with garden railways in California, where the climate is more conducive to such things. I helped one friend move a pretty good amount of dirt for cuts and fills on his railroad, which covered about tow acres.
Building a garden railroad is really not much different from building a real railroad in terms of the civil engineering standards you'll need to follow. Unless your yard is dead flat, it would really pay dividends in the long run to get a professional surveyor to layout out the route and grades for you, based on the plat plan you provide for him. If it's just going to be a a few hundred feet of track and you never plan to expand, you can probably do it by eye but, for anything bigger, getting a survey done will save a lot of grief when you find out your grades are too stiff for even the locomotive to make it up.
Being up in the mountains (what's your altitude?) presents some challenges but they can be overcome. You'll have to pay special attention to the grade. Not how steep the grade is, but the actual earthworks themselves. Your grade needs to be deep enough and compacted to resist frost heaving. The same is true for your ballast. You'll need a deep layer, like 6" thick, to keep the track in good shape, especially when you get periodic snow melt and runoff. Drainage has to be one of your main concerns since the combination of snow melt and summer thunderstorms can easily wash out section of your grade if you don't provide places for the water to go.
I don't know what kind of track you have but most G scale track will do fine in the winter as long as you don't let standing water drown the track. That's why drainage, grade, and ballast construction are so important. The most common type of track is brass, which will work fine after good cleaning in the spring. Copper prices have really gone up in the past several years so brass track is no longer cheap. Some modelers are switching to stainless steel and aluminum. If I was going to build in a harsh climate, I'd get aluminum. It's a great conductor of electricity, doesn't tarnish or weather, and looks more like steel rail without the drawbacks of steel. You could even run trains in the winter with an inch or two of snow as long as your engine has a pilot plow. That would look pretty neat.
Here's a great resource for you to check out:
http://www.btcomm.com/trains/primer/index.htm. They have a ton of articles for everything from making a track plan to landscaping. They also have some pretty good prices on garden railway supplies. Good luck and let us know how your plans are coming.
