I am traveling to Scotland in May 2017. I will be starting in Edinburg and would like to travel to Loch Lomond and possibly over to Cairngorms National Park. Is it possible to see these areas without a car - Just using Train and Bus?
For Loch Lommond - you may want to take a Rabbie's day trip. My family and I anchored for one week in Edinburgh then rode train and buses elsewhere for another week. We did 2 Rabbie's day trips from Edinburgh and really enjoyed them. One of them included Loch Lommond and I believe Stirling Castle.
How many days will you have, and where else -- if anyplace -- do you want to visit by rail outside of Edinburgh?
The UK national parks by definition are in more remote areas and therefore are not well served by public transport. Your holiday will be so enhanced if you hired a car, as your options are severely curtailed without one. Or take a tour. A car is a disadvantage in our cities, but essential elsewhere.