As others have pointed out, most of this is a good idea and already exists in the form of moving-block communications-based train control which is used all over the world.
The parts that don’t exist—traveling closer than the stopping distance and hitching/unhitching at speed—are not good ideas unfortunately.
You can’t travel closer than the stopping distance because you can’t guarantee that the train ahead will stop at its own achievable stopping distance. If you could, you could do as you describe, and have the next train start braking instantly when the one ahead brakes. But striking an unexpected obstacle and/or derailing can cause the train ahead to stop quite a bit more abruptly than that! If the train ahead suffers some kind of violent failure, it could leave sizable debris and/or track damage at the point of the event, even if most of the train continues along for some distance. So, even with moving-block CBTC, you must always be capable of stopping before reaching the current location of the train ahead of you.
Anyway it is not really necessary to travel so closely together, because the limiting factor of throughput is station dwell time. Sure you could have huge throughput on a line with no stations if you could theoretically travel at car-on-highway spacing, but if you have stations and they can only serve one train at a time, you won’t gain anything by doing this. You can improve this by having a platform with a track on each side so you can load two trains at once (and this is pretty common when dwell time is really long and throughput must be high, i.e. for intercity trains on a busy line), but that is the best you can do. You can’t really scale this to a bunch of platform tracks because you’ll start to have merging conflicts when rejoining the track.
The problem with unhitching at speed follows naturally from the problem with close following: the moment you unhitch, you are now following too closely behind another train, and you cannot stop in time if something bad happens to it.
> The problem with unhitching at speed follows naturally from the problem with close following: the moment you unhitch, you are now following too closely behind another train, and you cannot stop in time if something bad happens to it.
If you are unhitching though wouldn't your former train have been just as likely to hit whatever the now-unhitched component in front might be about to hit?
Basically the only scenario where I can see this realistically being an issue would be if the car(s) being unhitched were already on the edge of stability to the point that being unhitched was the straw that broke the camel's back and they then immediately crashed in front of the now separated unit following.
> So, even with moving-block CBTC, you must always be capable of stopping before reaching the current location of the train ahead of you.
And even if you say 'I don't care, I think the risks of using relative braking distance are acceptable', every set of points that needs to be moved over between trains effectively poses a stationary obstacle while it is in transit and thereby effectively still forces you to use absolute braking distance between trains.
Yeah, great point, especially since points are often the next bottleneck after station dwells (I’m thinking especially of BART’s Oakland Wye here). Close following really does not solve what matters for throughput.
The parts that don’t exist—traveling closer than the stopping distance and hitching/unhitching at speed—are not good ideas unfortunately.
You can’t travel closer than the stopping distance because you can’t guarantee that the train ahead will stop at its own achievable stopping distance. If you could, you could do as you describe, and have the next train start braking instantly when the one ahead brakes. But striking an unexpected obstacle and/or derailing can cause the train ahead to stop quite a bit more abruptly than that! If the train ahead suffers some kind of violent failure, it could leave sizable debris and/or track damage at the point of the event, even if most of the train continues along for some distance. So, even with moving-block CBTC, you must always be capable of stopping before reaching the current location of the train ahead of you.
Anyway it is not really necessary to travel so closely together, because the limiting factor of throughput is station dwell time. Sure you could have huge throughput on a line with no stations if you could theoretically travel at car-on-highway spacing, but if you have stations and they can only serve one train at a time, you won’t gain anything by doing this. You can improve this by having a platform with a track on each side so you can load two trains at once (and this is pretty common when dwell time is really long and throughput must be high, i.e. for intercity trains on a busy line), but that is the best you can do. You can’t really scale this to a bunch of platform tracks because you’ll start to have merging conflicts when rejoining the track.
The problem with unhitching at speed follows naturally from the problem with close following: the moment you unhitch, you are now following too closely behind another train, and you cannot stop in time if something bad happens to it.