Okay, let's take Jonathan Bartley (who's basically Zac Goldsmith with better PR) at his word and say a north–south high-speed line isn't the problem, HS2 is.

Let's make a Green-approved high-speed rail line! https://twitter.com/bbcmtd/status/1315208041743167488

First thing's first: construction is expensive! Let's not build much more than we really need to. So what's our first priority? Well, the London–Birmingham line, between two of Britain's largest cities, is rather congested. Let's draw a line between the two cities.

Now, we can look at where there are also capacity problems coming up: the lines into Manchester are rather congested too. Let's take the intercity services off the line to Stockport and onto their own dedicated line.

"But hang on!", you say, "that goes near Crewe, a major rail hub!"

And you're right! Let's add a station near there, and make the Manchester line a branch. That way, we can run services all the way to Scotland on the West Coast Main Line.

That's a good first attempt, but we need to be thinking of future-proofing. Where else could do with more capacity?

London to York seems a good bet; we've got two possibilities: another line, or branching off the London–Birmingham–Manchester line. Let's go with the latter.

And then, you think, "hold on, that goes pretty close to Leeds. Why not do what we did with Manchester, so we can run trains into Leeds, but also up to Edinburgh?"

Good idea! Let's do that on our map.

But just drawing straight lines means you go through the Peak District. That's a problem. Let's make a kink and send the line between Derby and Nottingham instead. We could even put a station there to serve both cities.

But trains don't go in straight lines. They can't. There are towns in the way, villages, hills and valleys. But we can go around them, of course!

We could also look at Birmingham to get even more using just a small amount of extra track.

Which gets you something like this.

Hold on, that looks incredibly familiar.

Wait a second...

Oh.

In the immortal words of David Allen Green,

Well.