Welcome to European Tribune. It's gone a bit quiet around here these days, but it's still going.
For Madrid-Sevilla, the explanation for 250 km/h is that that section crossed mountains. I don't know why they saved on the Córdoba-Sevilla section, maybe because it is a fertile valley with relatively high population density -> more NIMBY problems if they don't build it alongside an existing traffic corridor (they built the 200 km/h alignment along the old Iberian gauge line).

aside from economic reasons (e.g. less curvy = less maintenance)

Well, less curvy is also = more superstructures, and less maintenance only if operated at the same speed.

why build for 350-380 in the first place? If they up-permit it later then maybe using maximum design speed is not so wrong.

The speed raises in the past as well as the design speeds higher than initial line speeds had a variety of reasons, not all of which can be extrapolated.

  • First there is the maturing of a brand-new technology: say, going from 130 to 210 km/h was a great leap for Japan, but what they built was actually suited for even more by just increasing the mechanical tension of the catenary. Speed raises are obviously not so simple anymore.
  • Over the past few decades, there was a parallel improvement of track maintenance (the ability to keep tighter tolerances) and the running quality of vehicles (less strain on the tracks from random movements, lower propensity to derail), which allowed the elevation of speed limits in curves for example. There will surely be further optimisation, but these are no more the main obstacles (more later).
  • The last speed raises on Tokyo-Osaka and Tokyo-Aomori are related to the introduction of tilting high-speed trains. Tilting trains come around the problem of side accelerations felt by passengers, at the price of higher lateral track forces. But you rather don't do that on high-speed lines with ballasted track.
  • Sometimes line speeds are related to signalling. On France's first high-speed line, the less curved sections were uprated to 300 km/h after the signalling upgrade (of course with catenary improvement too). In Spain, the 300 km/h limit in operation is related to the lack of trust in the ETCS L2 signalling system, while all the rest of the infrastructure was in theory ready for 350 km/h.
  • As for China's 350 km/h Beijing-Tianjin and 380 km/h Beijing-Shanghai lines, as I wrote in my diaries, I think those are more a case of 'overclocking' than over-engineering (signalling inclusive).  
  • IMHO the  European lines with 350 km/h design speed have a somewhat similar issue, too. Here the thinking is that trains, rails, trackbed, catenary and signalling is replaced in a few decades, while lines will be there for a century or more, thus those replacements will allow the application of optimised technology without extra cost. However, I have growing doubts that this would go without a total track replacement (in particular for FS's 360 km/h plans). The Italian, French and Spanish lines with 350 km/h design speed have ballasted track, which for high-speed applications is inferior to slab track in several respects, in particular flying ballast due to vortices under the train. (This was an issue for the ICE3 in its original form on Belgian lines at much lower speeds.) I'm doubtful that further advances in underframe aerodynamics and tamping technology will suppress this problem enough, especially in snow.

*Lunatic*, n.
One whose delusions are out of fashion.
by DoDo on Sun Jul 17th, 2011 at 02:30:05 PM EST
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