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| Re: Latest updates: R-179 order saga, R-160 CBTC for (L) line & special reports ... (1135056) | |||
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Re: Latest updates: R-179 order saga, R-160 CBTC for (L) line & special reports ... |
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Posted by Stephen Bauman on Thu Jan 26 14:46:07 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 13:33:22 2012. Then how can it communicate where the block has moved to?There were two moving block systems in operation in NYC more than a century ago. Both were on the Brooklyn Bridge at different times. After 1908, fixed length blocks of 100 feet each were built. In order for a train to enter a block, both that block and the 6 blocks in front of it had to be clear. This enforced a minimum separation of 700 feet which was a restriction of the loading that the bridge could carry. This system enabled the bridge tracks to operate at 66 tph. Speed on the bridge was limited to 16 mph or 24 tph. This means that block moved every 4 seconds. The CBTC system has to allow for sporadic RF failures. The timeout is 2 seconds. This compares favourably with the moving block system of more than a century ago. The predecessor cable provided even better performance. It kept trains a fixed distance between one another regardless of their position on the bridge. The precision and accuracy of that train control system has not been equalled by newer technologies. It even provided rudimentary ATO. All without electricity. As a fixed block system has smaller and smaller blocks, the cost starts going up. NYCT, with its need to have trains running every 2-5 minutes on most lines, has a pretty robust signal system. Being able to get rid of such a "dense" system of fixed blocks should help the MTA reduce costs as the existing signals reach the end of their life cycles. You left out one important detail. Block length is determined by both desired headway (which you mentioned) and also by the expected distance between trains where the block is. When trains are operating at 30 mph at 40 tph, the expected distance between trains is approximately 4000 feet. Short block lengths are not required where trains are already separated by that much. Trains close in on their leader as they approach the station where the leader has stopped. Station approaches and platforms are where shorter block lengths are required for higher service level capacities. Block system designers have the luxury of placing shorter length blocks only where they will be needed. They can use longer length blocks where they are not needed. This allows them to design a system that minimizes the number of blocks (and cost) without compromising service level capacity. |
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