This rail route is considered to be one of the most scenic in Scotland. It certainly is a beautiful trip across the highlands of Scotland... perhaps not up to the grandeur of the Canadian Rockies by rail, but enchanting none-the-less.
The return trip can be done in one day, if travelling on a weekday. There's a train which departs Inverness at 8:55 AM, arriving at Kyle of Lochalsh at 11:17 AM. You'll have about 1/2 hour to walk around Kyle, which will be more than adequate as there's not much 'there' there. It's primarily the hopping-off spot for the Isle of Skye.
You'll depart Kyle at 11:52 for your return, arriving Inverness at 2:20 PM. So, even when the days are short your entire trip will in daylight.
Sit back, relax, and enjoy the scenery. Just be sure to take along a picnic lunch, as there's no food service on the train.