The route
LNER's East Coast Main Line runs the full length of England before crossing the border into Scotland, and at its fastest this is a direct four-hour-and-eight-minute run between two national capitals — with around 29 direct departures on a typical weekday, which works out to a service roughly every half hour for most of the day. That frequency, on a journey this long, is unusual: most routes this distance run a handful of times a day, not one every 30 minutes.
The approach into Edinburgh is one of the more dramatic arrivals on the network: the line runs along the Northumberland and Berwickshire coast for a stretch before turning inland, and the final descent into Waverley station drops you into a valley with the Old Town rising directly above the platforms.
What's in Edinburgh
Waverley station sits in a natural valley between the Old Town and the New Town, with Edinburgh Castle visible on its volcanic rock the moment you step outside. The Old and New Towns together hold UNESCO World Heritage status, recognising both the dense medieval closes and tenements of the Old Town and the ordered Georgian grid of the New Town built alongside it in the 18th century.
The Royal Mile runs the spine of the Old Town from the Castle down to the Palace of Holyroodhouse, and Arthur's Seat — an extinct volcano within the city, with a walkable summit and views over the whole of Edinburgh and the Firth of Forth — sits just beyond it. Every August, the city becomes host to the Edinburgh Festival Fringe, the largest arts festival in the world, when the population effectively swells with performers and visitors for the month.
Why this is the route the whole site is built around
Two capital cities, connected directly, on a schedule frequent enough that you can plan around it like a metro line rather than a once-a-day intercity service — this is the journey that makes the case for train travel over flying most convincingly, and it isn't an outlier picked to make the point; it's simply the best version of what every route on this list is trying to demonstrate.
Given the density of what's actually in Edinburgh, it's also one of the destinations on this list most likely to be under-planned for — treating it as a flying day trip from London leaves very little time for a city that easily fills two or three days on its own.