I'd second that - and add in the ECML north of Peterborough too, except the short bit between Morpeth and Berwick. A journey from London to Edinburgh is mind-numbingly tedious
That seems grossly unfair to me; I find most of the ECML between Doncaster and Edinburgh fascinating, and I travel the Newcastle to Donny stretch quite a lot: There's the plethora of power stations between Doncaster and York, the beauty of York itself and the tantalising opportunity to stop off for a pint or three mid-journey, being able to look across to the North Yorkshire Moors to the east and the Yorkshire Dales to the west along the racing stretch between York and Northallerton, the sinuous reverse curves between Darlington and Durham, the approach to Durham over the viaduct and the approach to Newcastle with the view of all the bridges. And north of Newcastle, most of the Northumberland coast is gorgeous: The sight of Alnmouth by the sea, running along the cliff tops around Berwick, and it all continues into Scotland too up around Grantshouse.
And from a railway enthusiasts perspective, there's also the freight yards at Doncaster Carr and Tyne Yard, the depots at Heaton and Craigentinny (And, to a lesser extent, York if 185s are you're thing), Doncaster works, and the tantalising prospect of glimpsing something special out of the National Collection sat out the back of the NRM or being shunted around the yards.
The stretch of line that I find most boring must be Sheffield to Doncaster or Wakefield. It always seems so grey and dismal, and progress seems uniformly slow. Completely uninspiring and tedious. It might also be something to with the journeys that I am usually undertaking: Typically Birmingham to Newcastle or vice versa, where I've been travelling for a long time, am getting a bit fed up with being sat on a Voyager, but am still a long way from my destination. And as I say, because of the need to fit the XC services amongst the slow local Northern services and freight traffic, the train never seems to be getting anywhere fast after the comparative romps along the MML and ECML on either side, and there's scenes of industrial desolation all around. I'm sure it'll feel even worse next month when I'll be doing Birmingham to Edinburgh that way, all on a Voyager.
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What?! That's crazy talk! I did the full length of the line for the first time early last year, with a coating of snow covering the beautiful Pennine scenery and a class 37 and 20 combo for haulage. Absolutely wonderful.

The five or so bottles of ale I'd consumed by that point probably helped as well.
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How about Stourbridge Jn - Stourbridge Town? The most interesting part of which is probably the use of a PPM instead of a 15x.
Nah, the journey's over before you even get a chance of considering being bored! Having a virtually unobstructed 360 degree view front, back and sides by virtue of Parry People Movers being employed on the branch is probably a welcome bonus though that helps relieve any sense of tedium.