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Post by paterson00 on Nov 25, 2009 5:35:15 GMT
Why does LUL have so many overlaps?
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Colin
Advisor
My preserved fire engine!
Posts: 11,346
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Post by Colin on Nov 25, 2009 6:20:08 GMT
Because we have a lot signals!
Each signal has it's own overlap as it's the overlap that provides the margin of safety.
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Post by citysig on Nov 25, 2009 9:48:21 GMT
Without overlaps, you would either have to hold more signals at danger to the rear of the train (which in any case would then form an overlap of sorts) or we would live in the constant danger of trains getting much too close for comfort.
Should the worst happen and a train passes a signal at danger, the overlap is there to ensure that, however fast the train passed the signal, by the time the brakes have done their stuff, it still will not have ended up in the back of the preceding train, or fouling a junction with the potential for collision.
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mrfs42
71E25683904T 172E6538094T
Big Hair Day
Posts: 5,922
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Post by mrfs42 on Nov 25, 2009 9:58:41 GMT
I think the question would be better answered in the sense of 'why isn't every overlap 183 metres long?' - because LU is not a mixed traffic railway - trains are of standard lengths and braking rates - each overlap can be optimised and precisely calculated.
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Post by railtechnician on Nov 26, 2009 3:12:17 GMT
I think the question would be better answered in the sense of 'why isn't every overlap 183 metres long?' - because LU is not a mixed traffic railway - trains are of standard lengths and braking rates - each overlap can be optimised and precisely calculated. Ah yes but just to complicate the issue a signal can of course have more than one overlap i.e. full speed or reduced speed to take account of the variances in frequency and headway of traffic at different periods in the timetable. |
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Post by paterson00 on Nov 26, 2009 4:29:43 GMT
I think the question would be better answered in the sense of 'why isn't every overlap 183 metres long?' - because LU is not a mixed traffic railway - trains are of standard lengths and braking rates - each overlap can be optimised and precisely calculated. Ah yes but just to complicate the issue a signal can of course have more than one overlap i.e. full speed or reduced speed to take account of the variances in frequency and headway of traffic at different periods in the timetable. Excellent now im really confused... Cheers guys |
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Post by railtechnician on Nov 26, 2009 7:48:30 GMT
Ah yes but just to complicate the issue a signal can of course have more than one overlap i.e. full speed or reduced speed to take account of the variances in frequency and headway of traffic at different periods in the timetable. Excellent now im really confused... Cheers guys Take somewhere like South Ealing on the Westbound Picc line. If the line ahead is clear through Northfields the signals will be green all the way so a train can attain full speed, thus a full speed (normal, if you will) overlap is allowed. However, there might be a train sitting in Northfields platform and as long as a train leaving South Ealing is travelling at a reduced speed it will be allowed to creep further ahead than it would otherwise do with a full speed overlap in force. The signalling is speed controlled using timing sections and because the train will travel at a reduced speed it will need less distance to stop in so there is also a reduced speed overlap which is shorter than the full speed overlap. There are also such things as shortened overlaps for platform starters in the tube because the train that left the station prior to the one about to leave will be accelerating away much faster than the one about to leave and will thus make (increase) headway to a safe interval by the time the train leaving the station is up to speed. |
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Post by citysig on Nov 26, 2009 9:29:45 GMT
... because LU is not a mixed traffic railway - trains are of standard lengths and braking rates. I beg to differ. Take a sample of my night shifts last week, and take Harrow as an example location, and the track there saw A-stock, Classes 165/166/168, Tamping machines, Battery Locos + Wagons, Track Recording Train and something else. What was it? Oh yes. S-stock ;D Now tell me that's not mixed traffic ;D But I know what you mean. On those tiny toy tube lines, then everything is more or less "standard" for the line and stock used. |
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Post by railtechnician on Nov 26, 2009 16:52:58 GMT
... because LU is not a mixed traffic railway - trains are of standard lengths and braking rates. I beg to differ. Take a sample of my night shifts last week, and take Harrow as an example location, and the track there saw A-stock, Classes 165/166/168, Tamping machines, Battery Locos + Wagons, Track Recording Train and something else. What was it? Oh yes. S-stock ;D Now tell me that's not mixed traffic ;D But I know what you mean. On those tiny toy tube lines, then everything is more or less "standard" for the line and stock used. Go on Met Control, tell them about the extra long engineers trains that run from time to time that don't fit in the sidings and are difficult to reverse. I don't think many observers note just how mixed the traffic can be at times! |
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Post by citysig on Nov 26, 2009 21:01:46 GMT
Or the extra short ones, that cause "trouble" with the new signalling. Ahem... I'll say no more...  ;D |
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Deleted
Deleted Member
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Post by Deleted on Nov 26, 2009 21:23:34 GMT
a silly question now.... whats a overlap? never come across that name before :s
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Post by 1938 on Nov 26, 2009 23:34:45 GMT
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Post by programmes1 on Nov 28, 2009 13:26:37 GMT
Or the extra short ones, that cause "trouble" with the new signalling. Ahem... I'll say no more... ;D Like a Tamping machine? |
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;D
