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Comment by londons_explore | Hacker News Reader

The main cause of heat is the inefficiency of the trains. All the brake energy is converted to heat. The motors and controllers are pretty inefficient too - modern electric motors+controllers are 95-98% efficient, whereas most tube trains are more like 40-60% efficient. For example, to go slowly, they simply put a big resistor in the circuit and waste 50+% of the energy!!

Perhaps not coincidentally, the inefficiency also has a massive carbon and budget impact... Or it would, if the train system had to pay for the electricity it used - which it doesn't! Trains in the UK are given free unmetered electricity, so there is no reason to conserve energy. Hot tunnels are just an extreme symptom of that.

If they had to pay for usage, at todays wholesale electricity rates, it would cost nearly a billion pounds a year. Which is ~20% of their revenue! I can see why they don't want to pay that bill!

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TFL does pay for electricity, but it pays for it as part of the Crown Commercial Service which aggregates procurement for many large public services in the UK. [1]

TFL used to have its own generating stations, which helped keep costs manageable and also meant the trains kept running during power cuts.

That ended around 2000. Now it has at least four separate connections to the National Grid, and is supposed to be moving to 100% renewables within the next decade.

[1] https://content.tfl.gov.uk/fc-20171205-part-1-item08-tfl-ene...

This is categorically not true.

In fact, the level of effort required to estimate electricity uses for multiple franchises on one physical section of railway has resulted in nearly all British rolling stock having built-in electricity metering.

Source on 40-60%? As some trains are from the early 70s, and others were introduced 10, 5 years ago, which are you referring to?

I'm referring to the oldest ones.

The new ones are much better, although they still recover very little braking energy.

>>put a big resistor in the circuit and waste 50+% of the energy

That Is Insane!!

Rail needs a complete rethink of design from the ground up. There are certainly vast efficiencies from rolling steel wheels on steel rails. But I've seen little evidence of light-weighting and indeed active resistance to steps like making cars using composites, which would save enormous amounts of energy and cost over the lifetime, as well as reduce maintenance costs. Yet their approach to everything seems to be sticking with an approach of 'brute force and ignorance'. Sure that works in the 19th century when you're just starting to put some carbon in iron and make it stronger than pig iron, but materials are sooo far past that now, yet it's still the same old approach. (&yes, I strongly support the Chesterton's Fence checking but this seems way beyond that)

I don't think it's anywhere near 50% that is wasted, since if I understand correctly the resistors are used only during initial acceleration.

But anyway, the affected trains are the oldest in use in Britain, from 1972 (Bakerloo Line, 36 trains) and 1973 (Piccadilly Line, 87 trains), out of 620 passenger trains across the whole system. New trains have been ordered and should be introduced in 2025.

There's been a 10 year delay, due in part to the 1990s attempt to privatize chunks of the system.

Yeah, it's insane because it's 100% not true.

Surprising then that a company sells resistors just for rail speed control applications:

https://www.cressall.com/applications/rail-traction-dynamic-...

(scroll down to "MOTOR STARTING AND SPEED CONTROL RESISTORS")

Thats not to "go slower" but to slow down:

Whenever possible regenerative braking is used. In this case the drive motors convert the kinetic energy of the train into electricity, which is fed back into the power supply and used elsewhere on the network.

If it cannot be absorbed by the system the energy may have to be dissipated with braking resistors. These braking resistors can be either mounted on the rail vehicle or as a fixed (trackside) installation connected to the third rail or overhead catenary via Automatic Assured Receptivity Unit (AARU) systems.

That section starts with "This was the traditional approach...". I guess the Bakerloo and Picadilly line trains from the 1970s might still use them on London Underground? Wikipedia mentions stuff like GTO thyristers and IGBTs on descriptions of stuff like the 1992 (Central line) and 1995 (Northern) stock trains, so the next oldest trains from the 1990s seem to have modern(ish) power control. And new trains are coming to the Picadilly line, and presumably must come to the Bakerloo at some point before the old ones fall apart.

That doesn't mean that's all that's used. It would also imply that it only has 2 speeds or something. Use your head. Or learn some basic electronics.

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