Oakville does have a few grades. I would tend to go with GWE. If referring to scenery well 60% of the Bakerloo Line is underground though there are plenty of gradients. TVL is also fairly flat terrain, a few gradients at the Saltburn end and for flyover tracks in the yard.
Peninsula Corridor actually has some rather steep gradients in the proximity of bridges and tunnels, and Rapid Transit is quite steep around the city tunnel portals, so GWE is a clear winner for me here since it has none of that whatsoever.
Great Western Express and Peninsula Corridor have to be two of the flattest (and most boring) routes in the game.
To be fair, Peninsula Corridor can become quite cool if you get into a nice groove (especially on the local services with stops at all the different stations). It does have a bit of variety, especially approaching San Francisco. And the Baby Bullet DLC makes the whole driving experience much more sleek and comfortable (with the stock loco, you’re essentially forced to worry about the brakes for 40min of your 60min service). Definitely not the worst route in the game on the boring -> exciting scale. Alongside GWE, I’d maybe throw WSR in there at the bottom end because it’s just too damn slow… a very uncomfortable speed for these diesel trains, which makes them very unrewarding to drive. Boston Sprinter is also quite bland and boring to me, it just lacks the passion many of the other routes have (and by the way: it’s also quite flat if I remember correctly).
Yes it's pretty much flat all the way through...except for the bit just before entering/after exiting Hamburg Hbf.
West Somerset is an heritage railway, mostly steam, and 25mph is the maximum speed permitted. That's why it's slow. I enjoy it because it's just down the road and I recognize everything. It needs steam to be added though.
The London to Bristol route was once nicknamed "Brunel's billiard table" for its highly graded alignment. Ronnie O'Sullivan could play a few games on the rails