It's a great ticket, although as Yorkie's said most of the traditional traction has gone
But still worth buying it. Stranraer, Oban, Mallaig, Kyle, Wick and Thurso are all corners of the network that are hard to reach, even from Glasgow and Edinburgh, so it's a good opportunity to see Scotland and get those lines done (if you're into coverage that is!)
There is a time restriction in place, though. You can't travel before 0915, although special easements are in place on the West Highland and Far North lines.
A couple of things to do:
1)
0821 Glasgow Queen Street-Mallaig
1345 Mallaig-Armadale (Calmac Ferry)
Buses to Kyle of Lochalsh
Train to Inverness
It's all doable in a day, and the buses and ferries are included! The only thing is that you have some tight connections, so work it out before you do it. If you do it in High Summer, you can do it all in daylight too 8)
2)
Fife Circle LHCS. The only locomotive hauled service trains in Scotland at the moment (excluding HSTs).
3)
Stranraer, WHL, Thurso/Wick, and all these silly out-of-the-way railway lines.
4) Remember that you're not just restricted to trains! Visit some Islands as well if you've got time; travel with Calmac is included in your ticket!
I'd basically try to do everything north of Perth/Dundee if I could. They're the northernmost lines in the UK, so it's hard to get to them from England. That's probably the best use IMO.