Thread by @SarahNobleLD

Let's talk a little about rail capacity, why HS2 is needed, and why any viable alternative to HS2 won't do anything to placate the a lot of the opposition.
There are, roughly speaking, two things that constrain the capacity of any train line: terminal capacity and headways.

Terminal capacity is how many trains can fit in any terminal station at one time. Headways are how many trains can be on a section of track over a period of time.

Terminal capacity is simple to solve: either increase the number of platforms, reduce the number of trains turning around, or run trains that take a shorter time to turn around. Typically commuter trains take about 20 minutes and intercity trains about 40 minutes to turn around.

We can see pretty well how the first two options can be implemented: Waterloo's former international platforms have given the station much needed breathing space. Crossrail and Thameslink, too, are there to provide a lot of breathing space for the BML, MML, GWML, and GEML.

In case you're wondering, Euston has the platform capacity for twenty trains to stop per hour. It's currently at 18 in the peaks.

Headways, though, are a different beast. Ideally, to get as many trains down a section of line as possible, you need trains going at the same speed and making the same stops as possible. It's how the Tube can have thirty trains per hour on a two-track line.

The West Coast Main Line, however? Over four lines, you get a maximum capacity of about 18 trains per hour. You have different rolling stock making different stops: commuter local and semi-fast trains going 110mph and long-distance express trains going 125mph.

But the real problem is that the WCML from Wembley also has to accomodate freight trains from Dover and London Gateway in Essex. And freight trains are slow. Like, under 70mph slow.

So this has a negative effect on headways. Even with dedicated fast and slow lines, the complex web of crossovers to allow semi-fasts to stop on the slow lines means that even the fast lines are beholden to the slows. You end up having massive fast trains crawl into London.

Enter HS2. It's a simple concept: We segregate the express trains to places in the North and Scotland from the other flows and build more platforms at Euston. This allows you to run many more trains down the multiple lines into Euston.

Preferably, as a country, we want to run more freight too; HS2 allows that! Unsurprisingly, you can run more intensive services down the fasts AND slows.

Of course, HS2 is not in a bubble. To relieve pressure on the Euston approach, you could also look at closing the Watford DC lines in favour of the Bakerloo and North London line. Or maybe send a couple of commuter trains an hour through the Crossrail tunnels.

It's an indisputable fact that we need more rail capacity between Birmingham and London. HS2 is actually the least disruptive method of doing so. Upgrade the existing tracks or add more. I hope you're ready for another decade of weekend closures.

Expand Euston without HS2? Just as disruptive, with less benefits.
Reopen the GCML? Congratulations, you've choked up the Metropolitan line and Marylebone station, and you don't go to Birmingham. Also, HS2 uses the GCML alignment in the Chilterns, so you'll still upset the NIMBYs

(As an aside, reopening the GCML could be a good idea, but to allow an alternate path from Leicester and Rugby into London and provide Daventry and Brackley with railway links. It'd be much more useful as a classic line.)

I get it, £50bn looks like a lot of money. But over twenty years, that's £2.5bn per annum, which is actually less than what Network Rail are spending on the classic network.

And yes, I know that the North is getting screwed over with transport upgrades. But that's not HS2's fault, that's Chris Grayling's fault. Don't blame vital infrastructure on the most incompetent Cabinet secretary in modern history.

If you cancel HS2, the money doesn't go elsewhere on the network. It just ceases to go into the railways budget altogether. It doesn't even go to other departments; austerity will still bite under a Tory government, HS2 or no.

And in a time when passenger numbers are increasing year-on-year, do you really want to cut the railways budget? Do you really want to cut capital investment, a significant driver of economic growth? It's just rank stupidity.

And, by the way, HS2 is better value for money than, say, Crossrail 2. Hell, most rail enhancements are better value for money than Crossrail 2. So one must ask why the Transport Secretary loves Crossrail 2 so much; is it because he lives on the line?

Another addendum: being supportive of HS2 doesn't mean you shouldn't be critical of some aspects! For example, I think the old Meadowhall route proposal was better than the Sheffield loop proposal; Similarly, Toton needs to have Nottingham trams stop there and go to Derby too.

Addendum the third: I see a lot of people complaining about "ballooning costs". That couldn't be further from the truth. HS2 has only had one real-terms rise in its budget: an extra £10bn added in 2013. The rest is either inflation, or the cost of the HS trains being added on.

Also, the £56 billion price tag includes a hefty £20 billion contingency for budget overruns. This means that the government think that it can be done for £36 billion, but they've put £20 billion aside for unknown circumstances, which they think has a 95% chance of being enough.

Addendum the fourth: environmentalism.

By and large, HS2 is designed to exact a modal shift from diesel-powered road traffic to electric-powered trains. Even with a grid that’s 60% fossil fuel, high speed rail is much much cleaner than even the best hybrid.

Yes, all those ancient woodland being “destroyed” sounds a lot. But to put it into context, that’s about 50 international football fields worth.

A service station on a motorway uses up about 20 fields worth.

HS2 is the only feasible proposal to shift freight onto the electric railway in a short enough time to avert catastrophic climate change.

If you cancel it, you’re just going to have to build more roads instead. And roads are an environmental disaster.

So why does the Green Party want to have to build more roads to deal with a gridlocked transport system, when a cleaner alternative already exists?

I wish I didn’t have to be so cynical to say that their HS2 policy changed under a leader who lived near Euston at the time, but…

Addendum the fifth: Technology.

A common argument is that HS2 will be obsolete by the time it opens, because some mythical technology will obviate the need for it.

Any rail industry observer will cringe at this, knowing the history of "technology means we don't need this!"

The last Labour government was, let's put it honestly, absolutely useless for transport if you didn't live in London. Only nine miles of track were approved for electrification whilst Blair was Prime Minister. Because they thought it wasn't needed. that it'd be obsolete.

This came to a head when the Labour government published the infamous "Delivering a Sustainable Railway" white paper, which basically said that we don't need electrification because hydrogen and biodiesel (cruelly dubbed "bionic duckweed") trains were just around the corner.

Ten years later, and we're still being told that hydrogen and biodiesel trains are around the corner. Supposedly they'll pair up greatly with Chris Grayling's Smart Digital Railway on the Blockchain Duck Cloud. (sorry @garius)

Incidentally, biodiesel and hydrogen are still incredibly dirty: biodiesel because of all the destructive farming of palm oil etc; hydrogen because our power grid is dirty.

Interestingly, the cleanest type of car to get in the UK would be a hybrid! But only just, and I digress.

So, what are the technological alternatives to HS2 that are "just around the corner"? Well, typically, three things get brought up: maglev, Hyperloop, and video conferencing. I'll deal with the alternate propulsion methods first.

Hyperloop, as it is, is an untested technology with dodgy economics. I daresay HS2 will be completed by the first commercial passenger Hyperloop services. Sorry, Musk fanboys, but it's true.

Maglev is more mature, but to date there is no long-distance Maglev line.

That will soon change, with the Chūō Shinkansen between Tokyo and Nagoya–180 miles–due to open in 2027.

But there's a problem. It's expensive. The current budget ¥9 trillion for the entire 400km to Osaka, or £62bn.

That's even more expensive than HS2, which goes further.

Also, there is a major problem with speeds in excess of 360km/h: acceleration. Distances required under the same acceleration to different speeds increase quadratically.

You don't want to be throwing up your lunch with a train going around a bend that it's not designed for.

To make the centripetal force comfortable, a train at 125mph needs to turn as if it's on a circle of radius 1 mile.

For 320km/h, the most possible operating speed, that goes up to 2 and a half miles.

For high-speed maglev (500km/h), 7 miles.

For Hyperloop (1200km/h), 40 miles.

Acceleration also makes accelerating and braking distances scale exponentially.

A standard intercity train takes 4 miles to get up to speed. HS2, ten miles. Maglev, 20 miles. Hyperloop, 115 miles!

So basically, there's no real advantage to maglev unless you're proposing an arrow-straight line between places that are more than 60+ miles away.

That knocks out much of the eastern leg of HS2, and quite a bit of the rest! After about 320km/h, you get diminishing returns.

Don't get me wrong, maglev looks cool, but it's impractical for high speeds in the UK.

So let's talk about video conferencing. Although the technology has progressed a lot, it won't be mature enough until 5G is rolled out at least, and it's still not a replacement for HS2.

There are simply too many jobs that require a personal presence, and while the average rail commuter is more white-collar, a lot of their jobs are those types where Work From Home doesn't work.

And let's be honest, who really does productive work at home?

Rail passenger numbers have increased year-on-year, and aren't forecasted to stop any time soon. That's why we're seeing so much rail infrastructure building. And HS2 is an essential part of that.

So no, technology won't make HS2 obsolete; if anything, it'll strengthen the need for HS2.