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| Re: Old LIRR Rockaway Beach Branch (1252767) | |||
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Re: Old LIRR Rockaway Beach Branch |
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Posted by Stephen Bauman on Thu Oct 10 12:35:13 2013, in response to Re: Old LIRR Rockaway Beach Branch, posted by SelkirkTMO on Wed Oct 9 20:21:08 2013. Let me present a dissenting opinion.As an artifact of the times when key-by was still permitted, there are several advantages. One is entering stations. If you're following your leader who is leaving the station and they continue to keep pulling away from you rather than you catching up with them, then you can enter a station on a red knowing that you can stop. In fact the IND did this on "station time" signals which would clear as a train was entering as another one was leaving. Did the operation you described reduce or increase headway? The follower who has keyed by will proceed at about 5 mph because there are no trippers to prevent a collision. The follower is 120 ft behind the leader and starts the instant the leader starts moving. It takes the follower 2 seconds to reach 5 mph and the follower has traveled 7 ft. Assuming 2.5 mph/sec for the service braking rate, the T/O will have to apply the brakes 7 ft before the mark. The train will travel for 600 + 120 - 7 - 7 = 706 ft at 5 mph. (station length + dist to leader - accel dist - brake dist). This will take 96 sec. The follower will have taken 100 seconds to come to a complete stop after the leader has started moving with keying by. Let's assume the follower does not key by. Instead the follower stops at the red signal that is 300 ft before the station entrance. The follower will wait for that signal to turn green before proceeding. He will then accelerate to 30 mph, enter the station at 30 mph and operate as if the leader did not cause the follower to stop. The signal the follower is waiting for will turn green, when the leader has travelled 600 ft (cleared the station). As mentioned in the previous post the leader will travel 264 ft in 12 seconds while accelerating to 30 mph. The leader will the will take an additional 8 seconds to clear the station, bringing the total up to 20 seconds. The follower will then accelerate to 30 mph in 12 seconds and 264 feet. The follower will take an additional 1 second to reach the station entrance @ 30 mph. From the previous timing post, the follower will come to a stop in the station in 20 seconds after entering it at 30 mph. The follower's total time from starting was 33 seconds. The total elapsed time from when the leader started moving is 53 seconds. So, it's 53 seconds without keying by vs. 96 seconds with keying by. Contrary to popular belief, keying by does not increase service levels - it destroys them. |
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