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I have a question.

Why aren’t shorter express trains with, perhaps, more automation and more spurs and sidings a feasible way to decrease travel times? Or, would such a modification work?

Is it possible to make high-speed rail more like personal rapid transit so that there is less deceleration and acceleration involved?

Maintaining constant speeds is so important for this mode that I question why the subject has not been addressed in the 50 years since high-speed rail was first introduced. The technology is seemingly there in order to make such an all-express system cost-effective.
 

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I didn't understand your second question. As for the first, I guess it has to do with economies of scale. 2-cars EMU would likely be very expensive to operate. Moreover, on more busy lines, there is the issue of safety distance between trains, which is quite high considering the high speeds they travel by.

Driverless operations would not be that difficult to implement, but I think we are not seeing train drivers going away anytime soon on HSR lines. There is the issue of emergencies and dealing with unexpected interferences on the tracks. Still, I'd like to see a completely sealed-off (as that even in the open it would be cover by transparent covers/protections) HS track system that would allow 100% driverless operations.

I still think, however, of the idea of making high-speed trains like airplanes, where you wear a seatbelt. Then, you could increase acceleration and deceleration (of course, new engines, locos and traction systems would be needed).
 

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Why aren’t shorter express trains with, perhaps, more automation and more spurs and sidings a feasible way to decrease travel times? Or, would such a modification work?
You mean "running more trains rather than faster trains"? Quite a few railways are indeed doing this.
 

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Why aren’t shorter express trains with, perhaps, more automation and more spurs and sidings a feasible way to decrease travel times? Or, would such a modification work?
In what way do you propose that sidings and spurs will decrease travel times? Sidings aren't used by passengers and spurs are a necessity on high speed rail as they are entirely grade separated. So this already exists.

Is it possible to make high-speed rail more like personal rapid transit so that there is less deceleration and acceleration involved?
Yes, by making the train more powerful. For passenger comfort it is not possible to accelerate to or decelerate from 300km/h faster than about 5 minutes so its not exactly going to have metro performance. Not even planes accelerate fast enough to make 500km/h a metro style service. Acceleration is the most energy-comsumptive part of nearly all transport. In general once you are going fast is it more efficient to keep going fast and arrange your service structure around that. As metro systems don't go fast there is little penalty in stopping a lot.

With respect to shorter trains, they are used somewhere but in general why would they be wanted? Many lines are nearing capacity as it is using 400m trains, or 2 200m ones joined together. To use shorter trains you'd need to remove some longer ones.
 
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