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| Re: SAS termini questions (127253) | |||
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Re: SAS termini questions |
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Posted by stephenk on Mon Aug 15 13:31:20 2005, in response to Re: SAS termini questions, posted by Rail Blue on Mon Aug 15 04:56:23 2005. "Very nice, Stephen. The only problem I see is that at least one of the following three will almost always be cheaper and have the same or better capacity:1) Passenger Loop 2) Non-passenger Loop 3) Partial relay in station, partial relay beyond station" Well, the flying junction terminus shouldn't be too much more expensive than a traditional 2 track/scissors crossover terminus. It would only need about 300-400m extra tunnelling. It would need 4 step plate junctions if in a deep level tube, but a scissors crossover would also need considerable cavern construction anyway. It would also need overruns (2x200m+), of which the length would be dependant upon the amount of train storage needed. Compared to the alternatives (all of which could handle 40tph):- 1) Passenger loop - would need approx 1000m+ of extra tunnelling for a 150m radius loop, more if 2 platforms are needed (which makes sense!). Also, passenger loops can cause further problems during service disruptions as there is nowhere to store trains off-line. Trains the wrong way round may cause problems. Building loops subsurface can also be problematic due to surface interference, and at deep level, the loops could be too sharp for TBMs! 2) Non-passenger loop - would need approx 1000m of extra tunnelling for a single track loop at 150m radius curve. More if two tracks were used (for off-line storage whilst the loop is in use, although alternately a crossover could be retained for quieter times). Also suffers from same construction problems as passenger loop, as well as potential problems caused by trains being the wrong way round. 3) Partial relay in station, partial relay beyond station - would need 3 platforms, and thus 50% or more station construction which would be very costly. It would also take up a wider site if built sub surface. Also, more difficult to operate than above options. * 150m radius curves used in examples, as that is the approx.radius needed to allow for 20-25mph platform entry exit speeds (which are the optimum speeds for the shortest possible headways through platforms) Of the three above terminus types, and the flying junction terminus, I much prefer the non-passenger loop for offering both high capacity, and flexible or reliable headways. However, where loop construction is not suitable (e.g narrow sites), I think that the flying junction terminus would be most suitable. Strangely, given the proven advantages of non-passenger loops, I know of none that have been built since the 1920s in heavy metro systems! ( Can someone correct me on this?) |
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