[avidiax]

> a comparable carriage fitted with pneumatic tyres could need as many as 20 wheels.

How does a bus get by with far fewer wheels?

I think the answer is that they are still building with the same weight as a train, rather than a bus.

That points out an unexplored engineering envelope for modern trains, made possible by newer technologies:

* Very light trains. Think lighter than road cars, since they don't need crumple zones or crash worthiness.

* Virtual coupling. Basically platooning on rails. Now the cars need to at most push/tow one other disabled car, so they don't need a beefy chassis to support towing long trains, coupling forces, etc.

* Homogenous cars. They all have traction motors, small batteries and sensors and compute. Think a low-range Tesla on rails.

* Autonomous control. Self-driving on rails. No operator cab. Since the train is now quite light, with a reasonable stopping distance, obstructions on the track can be potentially avoided so long as the sensors are adequate.

* Much faster acceleration and deceleration. With leaning, they could also corner faster.

* Probably intrinsically quieter, but now pneumatic tires would probably have reasonable life.

[jltsiren]

> Very light trains. Think lighter than road cars, since they don't need crumple zones or crash worthiness.

Crashes involving light rail are common in urban areas, because the trains share streets with cars. I don't see that going away as long as human-driven cars are allowed. And because passengers are often standing, the trains must be heavy to improve passenger safety in crash situations.

> Virtual coupling. Basically platooning on rails.

Modern designs typically have very long cars, with only 1, 2, or rarely 3 cars in a train. Longer cars increase passenger capacity and improve space utilization, because passengers can move around freely. They also allow busy passengers save some time by exiting from the right end of the train.

> Much faster acceleration and deceleration. With leaning, they could also corner faster.

Urban trains already limit acceleration and deceleration to improve passenger safety and comfort. Long-distance trains with sitting passengers and grade separation are another matter.

[avidiax]

> Crashes involving light rail are common in urban areas,

That's a good point. That doesn't mean that trains need to be as heavy as they are, though. Having a crumple zone and weighing 3x of non-commercial road vehicles would still be quite a bit lighter, and would provide safety.

> Modern designs typically have very long cars

I live in Switzerland, and it's common to have train combinations that are very long here, even with long cars.

> limit acceleration and deceleration

There are limits, of course, but they can be relatively high if you strictly control "jerk". That's where computer control comes into play, as it requires high driver skill to stop the train at the right position while limiting jerk.

https://en.wikipedia.org/wiki/Jerk_(physics)

[mikepavone]

> I think the answer is that they are still building with the same weight as a train, rather than a bus.

A non-trivial part of this difference is that train cars are generally bigger than a bus. Light rail is generally more bus sized and they are generally closer in weight (though still heavier).

> * Homogenous cars. They all have traction motors

Electric passenger rail systems generally already use EMUs which have a power unit per-car or per pair of cars.

> small batteries

I'm not sure how you're going to have a small battery in a bus-sized vehicle that needs to operate fairly continuously for a good portion of the day unless this is on a partially electrified ROW. EMUs with smaller batteries to serve such routes already exist FWIW.

I think there's a reasonable case to be made to adjust US passenger rail regulations to allow lighter cars (especially in the context of high-speed rail), but allowing pneumatic tires seems like a poor motivation for it.

[inferiorhuman]

  A non-trivial part of this difference is that train cars are generally bigger
  than a bus. Light rail is generally more bus sized and they are generally
  closer in weight (though still heavier).

Muni's articulated streetcars weigh about 100,000 lbs each while typically the max weight for a laden tractor trailer is around 80,000 lbs. Light rail is a marketing term, not indicative of the actual weight.

[mikepavone]

Not sure where you're getting the 100k lbs figure from. The Siemens S200 LRVs are supposedly 76k lbs as deployed in SF. Traditional "heavy" rail EMUs often exceed 100k pounds per car (Kawasaki M8s used by Metro North range from 97k to 144k lbs). The unarticulated New Flyer XT40 trolley buses that Muni also uses are only about 32k lbs by comparison, but they're only 40 feet long whereas the S200 LRV is 75 feet long. The articulated XT60s (60 feet long) would be a better point of comparison, but I can't find a weight for those.

[inferiorhuman]

SFMTA is saying 80,000lbs for the LRV2/3 and 78,000lbs for the LRV4 – I rounded it up to 100,000 because the last time I thought about their weight was when they were new (and their weight as a point of ire). Breda over promised and underdelivered and the LRV2s were quite a bit heavier than expected prompting a lot of teeth gnashing (and eventually a lawsuit if memory serves) from residents. Wikipedia pegs the New Flyers at about 45,000 lbs. BART's cars were in the 50,000 lb range due to their extensive use of aluminum.

Trains are heavy and often operate at faster speeds than buses, that's why they can get away with relatively few axles.

https://sfbayca.com/2016/05/05/new-muni-trains-on-schedule-f...

[TylerE]

That's been tried.

It did no go well, and was hated by both crews and the public. Rode extremely rough because it turns out it's a very different optimization needed.

https://en.wikipedia.org/wiki/Pacer_(British_Rail)#Analysis

[badcppdev]

Train crash worthiness is quite an interesting topic. Important to remember is that trains carry hundreds of people. So a crash involving a single train carriage can easily become a catastrophe.

https://www.youtube.com/watch?v=ewr-4TvG810

[alephxyz]

That's essentially a light rail system. Have a look at the Montreal REM.

[xnx]

Excellent ideas. In the US, metropolitan rail transit systems need a dramatic reimagining. They are expensive, slow, and often crime ridden. Would love to see their right of way put to better use.

[Zone3513]

We should rip out the train tracks, put down asphalt, and convert them to car tunnels, but to be used only by people driving one specific brand of car.

[xnx]

You joke, but a dedicated lane for self driving vehicles (either privately or publicly owned) could be lower cost and better utilization of the resources.