|
|
| Re: Latest updates: R-179 order saga, R-160 CBTC for (L) line & special reports ... (1135255) | |||
 |
|||
| Home > SubChat | |||
|
[ Read Responses | Post a New Response | Return to the Index ] |
|
||
|
Re: Latest updates: R-179 order saga, R-160 CBTC for (L) line & special reports ... |
|
|
Posted by Stephen Bauman on Fri Jan 27 16:28:55 2012, in response to Re: Latest updates: R-179 order saga, R-160 CBTC for (L) line & special reports ..., posted by J trainloco on Thu Jan 26 18:03:30 2012. So in the first paragraph, you say that it was a fixed block system! I understand what you're getting at, but at the end of the day, even though it was an extremely robust fixed block system, it was just that: fixed blocks.No, it's a moving block system. A fixed block system to guarantee 700 foot spacing between trains would have had 700 foot blocks across the bridge. The rule is that two red signals separate the leader from its follower. This means one block behind the leader would also have a red aspect. The distance between the leader and follower could vary between 700 and 1400 feet. The distance between leader and follower for the Brooklyn Bridge's moving block system was between 700 and 800 feet. That gap moved behind the the leader as he crossed the bridge. How does a train know its position in a CBTC based system? There are wayside beacons placed along the tracks. The carborne equipment reads the beacon as it passes it. The beacon may be an RF transponder, an RFID tag, an optical barcode or some other technology. It's supposed to be a lot cheaper install and maintain than an insulated joint. It was the failure to read these beacons that caused some of the early delays on the 14th Street Line installation. The system did not loose trains; the trains themselves did not know where they were. The internal control software does not measure train position in feet, meters, miles or microns. It measures them in integer block lengths as determined by the positioning of the beacons. The spacing of the beacons is determined by the desired maximum service level. Spacings between beacons of 100 to 600 feet is usual. While such spacings are adequate for collision avoidance, they are not for ATO where trains have to stop to a much tighter tolerance. Dead reckoning by counting wheel revolutions is used in addition to the beacons to make ATO stop near the same mark. You will note that the spacing between beacons is about the same as the spacing between insulated joints. Moreover, the algorithm used by the CBTC's "moving block" system are exactly the same as was used on the Brooklyn Bridge more than a century ago. The only possible difference besides implementation is one of scaling. Pardon my ignorance, but what system is this you're speaking of? A cable car? Don't knock the New York And Brooklyn Bridge Railway. The cars were 10 feet wide. The trains were longer than those on the El's on the Brooklyn and Manhattan sides. They also averaged 90 second headways over a continuous 24 hours with 80+ tph peak service after 1893. The cable was not only a partial ATO propulsion system but also ultimate moving block system for traffic control. As we used to say when I worked on the Apollo program, "if it works it's not sophisticated." You don't need to increase the amount of equipment to facilitate shorter blocks as you get closer to the areas where you will need shorter blocks. The CBTC equipment must be capable of handling maximum demand at all locations. Whether this is cost effective depends on the percentage of track is in or near stations and the CBTC equipment cost. An installation like the 14th Street Line should have favoured CBTC costwise because its large number of stations and short route length. That has not been the case. The benefit is even greater on some commuter or freight roads where the existing signalling might only allow for a much lower frequency, and transponders don't have to be placed so frequently. The way to handle long distance freight lines is to use GPS for determining the train's position. |
|
|
|