[londons_explore]

This still doesn't allow train spacing of less than the braking distance. Braking distance of trains is large (due to high uncertainty in track friction), which still means trains are typically 1 minute or more apart when moving fast.

As long as you have comms with the trains around, it's clearly possible to safely go less than the braking distance, as long as you can be sure of the behaviour of the train ahead, even in the case of an equipment failure (ie. in case of a power failure, it will not slam the brakes on, but decelerate at X m/s^2).

[thanzex]

I think that's not something that can be avoided, unfortunately. Any number of things could cause a train to suddenly stop. A mechanical failure, derailment, collision, a wagon could get detached... On roads we have millions of vehicles, carrying on average a very small amount of people, around 1.5. For efficiency sake we have accepted the risk of staying within reaction distance instead of stopping distance between vehicles.

It is a tradeoff between the safety of lives on board and traffic requirements that is relatively easier to accept when the average number of people involved is low against massive speed and efficiency gains.

The same cannot be said for trains though. Modern trains carry upwards of 1000 passengers, often at high speeds and without all of the safety and retention systems built into modern cars.

Having one or multiple trains with this large amount of people onboard be involved in a sudden catastrophic accident is possibly not worth the efficiency gained by thess than one minute separation.

Unfortunately we cannot just think about a normal scenario of simple deceleration

[sneak]

> On roads we have millions of vehicles, carrying on average a very small amount of people, around 1.5. For efficiency sake we have accepted the risk of staying within reaction distance instead of stopping distance between vehicles.

No. Unsafe drivers have illegally decided this, but in most jurisdictions it is your responsibility to stop your vehicle short of the one in front of you. You should be maintaining stopping distance from your vehicle to the one in front.

[lqet]

> You should be maintaining stopping distance from your vehicle to the one in front.

I am not so sure. In Germany, for example, the minimum required distance to the car in front of you is "speed in km/h divided by 2 in meters". So for 100 km/h, you are required to keep a minimum distance of 50 meters. I very much doubt that you can stop a car going 100 km/h within 50 meters.

[whartung]

You do t need to stop a 100 km/hr car in 50 meters because, barring that sudden brick wall appearing ahead of the car in front of you, they’re not stopping in 50m either.

The 50m is the reaction buffer, not the stopping buffer.

It’s also prudent to give more leeway to vehicles you can not see around to avoid the large truck swerving to avoid the refrigerator that just fell out of the truck in front of them problem.

[andruby]

That's interesting, in Belgium I think it's "2 seconds", or at least that's what was promoted recently on the radio.

You should be able to sing "Last night a DJ saved my live", which is a bit more than 2 seconds. I liked that, because it's one you can actually test for while driving.

I find it a lot harder to estimate 50m while driving at 100km/h

[lqet]

The reflector posts are always exactly 50 meters apart, which makes this relatively easy.

[corobo]

At least with cars they also have the option to swerve away from the chaos if the stopping distance is too short.

Not to disagree, just throwing in a variable that trains don't have

[thanzex]

Not really. In my country, at least, it is explicitly stated in driving theory manuals to maintain at least one reaction distance between the car in front of you.

For a car traveling at 100Km/h the stopping+reaction distance would mean more than 130m, which is a quite large and possibly impractical for higher traffic scenarios.

[mezmor]

Aren't braking distances so large because of the massive amounts of mass in motion? Inertia, etc. Even high certainty of track friction wouldn't change that.

Also, I don't think it's ever safe to be closer than braking distance because the bahavior of the train ahead is never guaranteed. There's always a chance somebody parks their car on a rail crossing or some other sudden stopping-event occurs.

[TylerE]

No, they’re very long because steel wheel on steel rail has much much much lower coefficient of friction than rubber on asphalt. If they brakes any harder they wheels would just lock up and skid, both running the wheels AND taking longer to stop than a normal braking application.

[lqet]

It is not safe. If your train crashes into a truck, or derails, or explodes, all trains behind will crash into it if they are nearer than their own breaking distance. This is not possible in a classic block system: the block is freed only when the last carriage of the blocking train has left the block.

[TylerE]

That’s a metric crapton of ifs. I would never get on such a system, it is fundamentally unsafe.

[andbberger]

this is decidely a feature and not a bug. CBTC systems work by continually updating track leases to the last known position. this allows for high throughput while safely avoiding collisions in case of catastrophe.

highway separation is unacceptable for anything other than trams (which are slow and have track brakes). the path to safely increasing throughput is to increase braking performance and minimize dwell time. existing systems eg paris line 14 and the tokaido shinkansen have very high throughput.

transit projects would look even better compared to highways if they had to be safe too!

[pixiemagic]

What happens if there's some other reason to brake, such as an unreported fallen tree on the line ahead?

[rjmunro]

As soon as one train brakes, the train behind should automatically brake. The separation distance can be maintained. The problem is if the first train hits an object or derails, this might cause it to slow down faster than the brakes would have done, and the following train may not have time to stop.

[M95D]

What happens if the train in front is a passenger train (a passenger pulled the alarm handle) and the train behind is 30 cars loaded with coal, steel beams or diesel fuel?

[londons_explore]

You can still brake - you just need (automatic) agreement of all trains behind you to do so.

You aren't allowed to brake 'as much as possible' anymore - instead the best you can achieve is 'the best the worst of the trains behind me can achieve'.

[garaetjjte]

If one train derails for some reason, then every single train on the line crashes in the greatest pileup ever. Great.

[londons_explore]

Thats already the case for carriages within a train.... And people are fine with it.

We're also fine with the risk of a derailment onto a neighbouring track with traffic going the opposite direction. We could have tech which detects that, but we do not.

[lqet]

> And people are fine with it

The thing you are missing is compartmentalization. A modern high-speed train can hold around 1,000 people. This is the maximum number of people that could die in an accident (ignoring trains on other tracks or people next to the tracks). What you are proposing is essentially a train of infinite length, with virtual (software) coupling between groups of carriages. But then there is no limit to the maximum number of deaths in an accident. If you have 50 high-speed trains all traveling in this virtually coupled manner, a single accident may cause 50,000 casualties. People would not accept this, and no sane policy maker would allow it.

Note that if you consider derailments into neighboring tracks, you still have an upper limit of 2 times maximum train capacity, in our example 2,000.

[corobo]

A train derailing onto another track would be a hell of a derailment

Take this incident for example https://www.bbc.co.uk/news/uk-england-surrey-68466494

Imagine that then resulted in "the following train then slammed into the derailed train, having been unable to stop in time"

[rjmunro]

It happened at the Clapham Junction rail crash in 1988: https://www.bbc.co.uk/news/uk-england-london-46509473

A total of 3 trains were involved.

[blibble]

one of the original tube line ATO systems allowed a train to enter a platform as another was departing

people did not seeing this and the system was modified to not do it

[danhor]

You could also characterize the maximum braking ability of the train in front of you and the minimum braking ability of your own train and determine the needed distance for any given speed based on that. This would of course be more complicated and probably be solved by only including broad categories of trains (i.e. only two sets of values for either passenger trains or freight trains).

[andbberger]

this is how CBTC works

[TylerE]

You also would need to consider the braking ability of the train BEHIND you.