Trains
Large network & reasonable prices
Scotland's trains connect all it's main cities and ferry hubs and it's perfectly possible to plan an entire trip using only the train.
Many routes are also incredibly scenic; with the best being the line between Glasgow and Oban, and between Glasgow and Mallaig (a route also served by the Jacobite Steam Train); as well as stretches of the route between Perth and Aviemore.
Tickets
Britrail sells passes to overseas customers but unless you want a pass for the whole of the UK you'll save money by simply buying one over the counter at a any staffed Scottish station. On offer are the Spirit of Scotland Travel Pass (4 of 8 days/8 of 15 days; £149/189) which covers all of Scotland. There is also a ScotRail Highland Rover ticket (4 of 8 days; £95) for just the Highlands and a Central Scotland Rover (3 consecutive days; £55) which covers Edinburgh, Glasgow, Stirling and all the bits between.