4 aspect, compromise of principles?

4 aspect, compromise of principles? - Printable Version

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4 aspect, compromise of principles? - fil - 19-05-2011

Hi,
I have recently read :
"4 aspect signalling, reduced overlaps and permissive working are all examples of signalling principles made flexible to meet operational needs."

does anybody know how exactly 4 aspect signalling falls into this category?

thanks
fil

RE: 4 aspect, comprimise of principles? - PJW - 20-05-2011

(19-05-2011, 08:58 PM)fil Wrote: Hi,
I have recently read :
"4 aspect signalling, reduced overlaps and permissive working are all examples of signalling principles made flexible to meet operational needs."

does anybody know how exactly 4 aspect signalling falls into this category?

thanks
fil

I guess in the historic context that the warning to brake to a stop signal had always been given by one signal at braking distance. Hence it must have been a bit radical to have proposed a 4 aspect system whereby a waring to brake is given at a signal to stop not at the next but the signal beyond that and to have "overlapping" braking distances for alternate signals. With the perspective of hindsight, we just accept the philosophy but it probbaly didn't seem like that at the time of introduction.

RE: 4 aspect, comprimise of principles? - reuben - 20-05-2011

Take 1

I seem to recall reading, but wasn't around at the time that:

Semaphore signalling always used an overlap of 1/4 mile.

This figure was derived by experiment, but was ultimately an arbitrary figure.

Once this arbitrary figure got written down as being a "standard", anyone who suggested using a distance even a yard less than the written down figure was crucifed as a heretic

When 2, 3 and 4 aspect colour signalling came along, an interim arrangement was that 2 aspect used a 400 yard overlap, 3 aspect used a 300 yard overlap, and 4 aspect a 200 yard overlap, the thinking being that in the latter cases, the driver is given more frequent updates about the state of the line ahead, and so the likely scale of an error will be reduced.

- hence it could have been said that the heretically short 200 yard overlap of 4 aspect signalling was a compromise of signalling principles to meet operational needs.

Take 2

Signal engineers often tend to think that we're so important that we are self justifying, and those trains are just an inconvenience.

Let's just remember that if there weren't any customers, there would be neither trains nor signal engineers.

so lets's amend the statement;

"Restrictive Signals, overlaps and inflexible signalling are all examples of the commercial train service delivery being compromised by fixed signalling rules"

Let's see who gets annoyed by the above statement!

RE: 4 aspect, comprimise of principles? - PJW - 20-05-2011

(20-05-2011, 04:47 PM)reuben Wrote: Take 1

I seem to recall reading, but wasn't around at the time that:

Semaphore signalling always used an overlap of 1/4 mile.

This figure was derived by experiment, but was ultimately an arbitrary figure.

Once this arbitrary figure got written down as being a "standard", anyone who suggested using a distance even a yard less than the written down figure was crucifed as a heretic

When 2, 3 and 4 aspect colour signalling came along, an interim arrangement was that 2 aspect used a 400 yard overlap, 3 aspect used a 300 yard overlap, and 4 aspect a 200 yard overlap, the thinking being that in the latter cases, the driver is given more frequent updates about the state of the line ahead, and so the likely scale of an error will be reduced.

- hence it could have been said that the heretically short 200 yard overlap of 4 aspect signalling was a compromise of signalling principles to meet operational needs.

Take 2

Signal engineers often tend to think that we're so important that we are self justifying, and those trains are just an inconvenience.

Let's just remember that if there weren't any customers, there would be neither trains nor signal engineers.

so lets's amend the statement;

"Restrictive Signals, overlaps and inflexible signalling are all examples of the commercial train service delivery being compromised by fixed signalling rules"

Let's see who gets annoyed by the above statement!

and
Not rising to the bate for "take 2" (I broadly agree with you and whereas there may have been an argument to avoid the complexity of speed signalling when implemented by a large number of permutations of coloured lights feel that the UK should have adopted in a similar form to the Dutch or Swiss modernised versions in which the speed info is give by a simple numerical display). For "take 1" I think you could be right, but actually I believe that the length of overlap in those days may also depended upon gradient as well (?). Similarly a semaphore Block Clearing Point can be reduced to 200yds when the distant is a colour light rather tha a oil lit semaphore (I think that even a torch bulb lit semaphore counts i this cotext as a colour light) but not sure if this more a more recebt easement

RE: 4 aspect, compromise of principles? - fil - 28-05-2011

thanks guys,

i will take the take 2 comment on board, and start to be a little more open minded when it comes to the operational railway.

my thoughts, is perhaps the principle of yellow=prepare to find the next signal at red, is been reffered to.

thanks