The route
The Caledonian Sleeper, currently operated by Serco under the Scottish Government-owned brand, is a genuinely different kind of train from everything else on this list: it departs London Euston in the evening, travels overnight, and arrives in the Scottish Highlands at breakfast time. Sleeper services on this corridor date back well over a century, to the late 19th century, making this one of the oldest continuously-operated types of rail travel in Britain.
The Highland portion of the train physically splits during the night: carriages bound for Inverness, Fort William and Aberdeen are coupled together at the start of the journey and separated at Edinburgh in the small hours, each continuing on to its own Highland destination while passengers sleep through the shunting.
What sleeper travel is actually like
Accommodation ranges from seated carriages up through shared and private berths, with the private cabins including a bed, a washbasin, and in the higher classes an en-suite shower and toilet. A steward service and an on-board breakfast are included in most cabin fares, so the ticket functions as both transport and a night's accommodation, arriving refreshed into the city rather than having spent the night in a hotel room you'd otherwise need to book separately.
Because it's an overnight service, it's worth booking a cabin (rather than a seat) well in advance if actual sleep is the goal — the private berths sell out faster than the seated option, particularly in peak summer months.
What's at the end
Inverness is known as the capital of the Highlands, and functions as the natural base for exploring Loch Ness (a short bus or drive away) and the wider Highland landscape of glens, lochs and mountains beyond the reach of any other train on this list.
It's also, simply, the only journey covered here where an overnight sleeper is a realistic option at all — every other route is short enough to make more sense as a day service, which is exactly what makes this one worth calling out as a distinct kind of trip rather than just a longer version of the others.