A few comments:

About Japan: Things are different if you have a train every 2 minutes; in such a case, 15 seconds delay can already destabilize the system. Or stopping not exactly at the stopping mark.

However, speeding is essentially a no-no, even in Japan.


About protecting a curve against overspeed. This is possible with conventional signalling systems in most countries, and it gets a little bit easier with ETCS (even Level I). However, it will require a linear supervision of the train, which is more advanced but also delicate than the typical point to point supervision. Germany has a rather extensive use of linear supervision (LZB, Linienzugbeeinflussung), on the high speed lines, but also on the high-density lines (for example the S-Bahn tunnel in München (with, if I am not too off, more than 30 trains per hour and direction). There are other systems where only some 100 meters are equipped with linear supervision, just before the point of danger.

But linear supervision is rather expensive, and requires in many cases a complete rebuild of the towers and/or control centers. Again, it becomes a bit more affordable with ETCS. But no matter which system is chosen, planning and implementing takes its time.

Many conventional systems can convey certain speeds (or signal aspects which then translate to speeds), but chances that these speeds are what is best for that particular curve are rather low. Which means serious upgrading of the signalling system.

As said, there are new systems which would have a pseudo-continuous supervision of the location of the train, such as ETCS Level 2. In this case, establishing the maximum speed before points of danger becomes relatively easy. But that's something you don't buy from the supermarket, and planning and implementing takes its time as well.