Following on from Grifter's thread ( http://www.modelrailforum.com/forums/index...9445&st=120 ) but keeping the information seperate/distinct...
Posts 131-133 dealt with the connections at the outer ends of a platform loop worked by one Box - which would be roughly half way between the ends. The question raised was whether the exit end would be worked at the same time as the entry end by the same lever. Basically the example we are dealing with uses mechanical locking - which means that the answer is "no". This is for two basic reasons:-
When powered point mechanisms are used the distances can be greater and, clearly, the power replaces the Signalman's weight and muscle on the lever... but the impact on shunting would not be resolved... Plus it would produce some rather interesting effects on the interlocking and the proving that the road is set correctly.
Something we should consider here is that a lot of the early point mechanisms were simply installed into layouts that already existed, that had been set out for manual working and interlocked for the mechanical frame (as part of it). So that all the point mechanisms would do would be to replace the rodding and remove part of the Signalman's work load. (That work load could be put back if mains electric wasn't available and the motors had to be shifted by hand generated power).
With suitable, reliable electricity the ability to use electric locking arrived. This could and did simplify how things were interlocked - but - this was usually added to a pre-existincing mechanical frame. One thing about mechanical frames - you do NOT want to alter anything unless you really have to -not even to "simplify" things. Mechanical interlocking is very much a matter of "if it's working - DO NOT mess with it!" Therefore additional locks tend to be added but little is usually taken away. (This actually results in some frames that have lost functions - such as route connections - ending up with colour over White levers... Usually Blue (i.e. a lock) over White... This is because the locking has been retained to avoid messing with the frame but the original function of the lever (to do a job outside) has gone.
Then, when there is money to invest and a scheme has been agreed, there is the possibility of going to a Relay Interlocking - and then onto the next step of a Route Relay Interlocking. These will nearly always use Power Points and Colour Light Signals for everything - especially a Route Relay Interlocking. There is somewhat more chance of either of these combining the two ends of a loop - except for the fact that if the ends are worked together they not only have to be proved Reversed together to confirm the route for the signal - but - if a train is going to be held in the loop they will have to be confirmed set back to Normal together - AND the route beyond the exit will probably have to be cancelled out of the system... As a general principle the people designing early route systems hated to provide for cancelling a route - some Boxes couldn't do it at all - but would have to call an S&T technician to give a release. (This occured with Dorking North Box for one).
Once we head toward modern stuff - and on to Solid State Interlocking we can tell the system to do all sorts of things... But - while a route may automatically set through a route the ends will still be kept seperate and independent within the system.
So - that deals with a little of the way things are locked up and why.
Then we can look at how we actually get trains into the loops...
Just in case a train fails to stop when it should on entering a loop it is preferred for it to have somewhere to go to - other than into a risk of hitting anything else on the track - specifically - on the Running Lines - and Particulalry on Passneger Carrying Lines.
This means that beyond the signal protecting the exit end of a loop there will be an "overlap" are provided. As with all things electric this tends to be more of a modern development - the earlier railways worked more on the basis that the thing should stop where it was supposed to... However - they didn't always stop... There were (at least) three solutions to this...
So - we have now introduced speed restrictions...
Loops lines are not usualy (if ever) entered through high speed points - so there has always been a strong tendency for there to be a speed restriction on the points going into a loop - and another one for the points at the exit end. There is a simple bit of physics that comes into play here - if a train exceeds the speed limit on either set of points - it is at risk of falling off anyway.
We therefore make sure that train crews (the Guard is responsible as well) are aware of where they are and what the speed limits are. Footplate crew have an added incentive to get this right - they will be the first to hit anything - it's called "aversion therapy".
When the route into a loop has a more restricted speed than the Running Line it will have either a lower signal arm as a semaphore - using the Position Value system --- This can also be applied to Colour Light arrangements where multiplehaeds are used instead of Lunar Lights (feathers). More often Colour Lights use Feathers (Lunar Lights for diverging routes...
With lunar lights a single route to the left will usually have the feathers at position 1 - which is at about the ten-thirty position of a clockface. If the route is much slower this single set of feathers may be put at position 2 - the nine-o'clock position as an indication of the decreased permitted speed... And this can be made more severe by using the position 3 - which is about the seven thirty position of the clockface.
This adds up to providing signals that visually tell train crew to slow down by the way that they are configured.
It is also possible to add to this by using a different type of signal for the diverging route.
With semaphores this is usually done by using a smaller arm than the main route. This smaller arm will be set lower than the main route arm - so that Position Value is added to by the different size of the arm. The aspects will also be smaller. Different railway companies did this in slightly different ways.
With Colur Light signals there are a number of ways of achieving the same end. This can include using shunt type disc signals (usually motor or solenoid operated) or various options of small signal. The most common design to have spread all over the country has been the Position Light Signal - these will normally show no aspect except when the diverging route is set and cleared. (When cleared they will show two white lights at a 45 degree angle upward to the left - under the Red signal of the main aspect which will not go out.
So - that's what can be done with the signals themselves...
The next thing is to control the trains approach by when the signal is cleared.
For mechanical signals (semaphores) this means that the watching Signalman will keep the signal into the loop at Stop/Danger until he sees the approaching train is running under control and able to stop dead at the signal - or that the train has actually stopped dead at the signal. Only then will he clear the route into the loop - for which he will usually have already Reversed the points. This control of the train before entering the loop should normally mean that the train doesn't slide out of the exit end.
With Colour Light Signals (and with Motorised semaphores - although it doesn't tend to be done with them - that I know of) it is possible to achieve the same control by slowing to a Red signal by arranging the signal to be "Approach Controlled". When that is done the signal into the divergence will not clear unless the train is detected to either be travelling sufficiently slowly or at a stand.
That was good wasn't it?
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Meanwhile - back at aversion therapy...
It is in the interest of train crews, particularly those at the front, to not crash into things... So they will manage the speed of their train appropriately so that there is a minimal chance of it over-running the signal protecting the exit from the loop.
When a train is an unfitted or part fitted goods (i.e. it has no continuous automatic brake or is only provided with continuous automatic brake at the front end - a "fitted head" - the crew will be extra wary about running toward any signal that is against them. Working together the footplate crew and guard will slow the train as they approach the Red signal. Doing this they will keep all the couplings stretched out as much as possible... So - they are likely to slow right down and then draw toward the signal - because the slowing part will tend to let the slack in the couplings "run in" so that they drop slack - and then the action of drawing forward (gently) will draw them all up and tight again. (In fact the guard will apply his brakevan screw brake as the train slows so that the van at the back will hold back the train and tend to keep the couplings from slacking...
All that lot means that when the signal into the loop is against a train - and even when it is cleared - the crew will be working to have and keep their train under control so that, having slowed down, they will then draw forward to where they want to stop.
I am now going to have a coffee!
![]()
Posts 131-133 dealt with the connections at the outer ends of a platform loop worked by one Box - which would be roughly half way between the ends. The question raised was whether the exit end would be worked at the same time as the entry end by the same lever. Basically the example we are dealing with uses mechanical locking - which means that the answer is "no". This is for two basic reasons:-
- 1. The amount of point rodding - and therefore the effort to shift it - would be too great.
- There is also a potential issue of the distance from the lever to the two points. Most of the time we can reckon that a Facing Point will not be more than 200yards from the lever and a Trailing Point not more than 300 yards - unless we are on a very busy, and updated, main line post c1935. This would mean that the maximum train length within the points (outer ends), the track that widens around the platforms and the loco and carriages would have to add up to less than 500 yards - 16x60ft coaches = 340 yards + loco about 30 yards for a large pacific - leaves us only 130 yards for the track and points at each end - which would probably be pushing it...
(In fact - with much shorter trains the passing loops on Single Lines with a Box in the middle do not have the opposite ends linked on a mechanical frame (there are other reasons for this as well).
- There is also a potential issue of the distance from the lever to the two points. Most of the time we can reckon that a Facing Point will not be more than 200yards from the lever and a Trailing Point not more than 300 yards - unless we are on a very busy, and updated, main line post c1935. This would mean that the maximum train length within the points (outer ends), the track that widens around the platforms and the loco and carriages would have to add up to less than 500 yards - 16x60ft coaches = 340 yards + loco about 30 yards for a large pacific - leaves us only 130 yards for the track and points at each end - which would probably be pushing it...
When powered point mechanisms are used the distances can be greater and, clearly, the power replaces the Signalman's weight and muscle on the lever... but the impact on shunting would not be resolved... Plus it would produce some rather interesting effects on the interlocking and the proving that the road is set correctly.
Something we should consider here is that a lot of the early point mechanisms were simply installed into layouts that already existed, that had been set out for manual working and interlocked for the mechanical frame (as part of it). So that all the point mechanisms would do would be to replace the rodding and remove part of the Signalman's work load. (That work load could be put back if mains electric wasn't available and the motors had to be shifted by hand generated power).
With suitable, reliable electricity the ability to use electric locking arrived. This could and did simplify how things were interlocked - but - this was usually added to a pre-existincing mechanical frame. One thing about mechanical frames - you do NOT want to alter anything unless you really have to -not even to "simplify" things. Mechanical interlocking is very much a matter of "if it's working - DO NOT mess with it!" Therefore additional locks tend to be added but little is usually taken away. (This actually results in some frames that have lost functions - such as route connections - ending up with colour over White levers... Usually Blue (i.e. a lock) over White... This is because the locking has been retained to avoid messing with the frame but the original function of the lever (to do a job outside) has gone.
Then, when there is money to invest and a scheme has been agreed, there is the possibility of going to a Relay Interlocking - and then onto the next step of a Route Relay Interlocking. These will nearly always use Power Points and Colour Light Signals for everything - especially a Route Relay Interlocking. There is somewhat more chance of either of these combining the two ends of a loop - except for the fact that if the ends are worked together they not only have to be proved Reversed together to confirm the route for the signal - but - if a train is going to be held in the loop they will have to be confirmed set back to Normal together - AND the route beyond the exit will probably have to be cancelled out of the system... As a general principle the people designing early route systems hated to provide for cancelling a route - some Boxes couldn't do it at all - but would have to call an S&T technician to give a release. (This occured with Dorking North Box for one).
Once we head toward modern stuff - and on to Solid State Interlocking we can tell the system to do all sorts of things... But - while a route may automatically set through a route the ends will still be kept seperate and independent within the system.
So - that deals with a little of the way things are locked up and why.
Then we can look at how we actually get trains into the loops...
Just in case a train fails to stop when it should on entering a loop it is preferred for it to have somewhere to go to - other than into a risk of hitting anything else on the track - specifically - on the Running Lines - and Particulalry on Passneger Carrying Lines.
This means that beyond the signal protecting the exit end of a loop there will be an "overlap" are provided. As with all things electric this tends to be more of a modern development - the earlier railways worked more on the basis that the thing should stop where it was supposed to... However - they didn't always stop... There were (at least) three solutions to this...
- 1. Drop an errant train in the dirt - through Trap Points or a derail.
- In principle any train carrying passengers should have 440 yards of track to wander off into - but - (a) this being impractical in most cases a long sand drag would be utilised instead and speed restrictions applied - and - (
this length has tended to be reduced to 200 yardsw as a more universal overlap provision.
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3. Route an errant train back out onto the Running Line it had just left... Which would mean that while the train was going into the loop the line beyond the exit had to be kept clear for it to over-run into - through points that were Reverse - but - worked seperately from the entry points - In principle any train carrying passengers should have 440 yards of track to wander off into - but - (a) this being impractical in most cases a long sand drag would be utilised instead and speed restrictions applied - and - (
So - we have now introduced speed restrictions...
Loops lines are not usualy (if ever) entered through high speed points - so there has always been a strong tendency for there to be a speed restriction on the points going into a loop - and another one for the points at the exit end. There is a simple bit of physics that comes into play here - if a train exceeds the speed limit on either set of points - it is at risk of falling off anyway.
We therefore make sure that train crews (the Guard is responsible as well) are aware of where they are and what the speed limits are. Footplate crew have an added incentive to get this right - they will be the first to hit anything - it's called "aversion therapy".
When the route into a loop has a more restricted speed than the Running Line it will have either a lower signal arm as a semaphore - using the Position Value system --- This can also be applied to Colour Light arrangements where multiplehaeds are used instead of Lunar Lights (feathers). More often Colour Lights use Feathers (Lunar Lights for diverging routes...
With lunar lights a single route to the left will usually have the feathers at position 1 - which is at about the ten-thirty position of a clockface. If the route is much slower this single set of feathers may be put at position 2 - the nine-o'clock position as an indication of the decreased permitted speed... And this can be made more severe by using the position 3 - which is about the seven thirty position of the clockface.
This adds up to providing signals that visually tell train crew to slow down by the way that they are configured.
It is also possible to add to this by using a different type of signal for the diverging route.
With semaphores this is usually done by using a smaller arm than the main route. This smaller arm will be set lower than the main route arm - so that Position Value is added to by the different size of the arm. The aspects will also be smaller. Different railway companies did this in slightly different ways.
With Colur Light signals there are a number of ways of achieving the same end. This can include using shunt type disc signals (usually motor or solenoid operated) or various options of small signal. The most common design to have spread all over the country has been the Position Light Signal - these will normally show no aspect except when the diverging route is set and cleared. (When cleared they will show two white lights at a 45 degree angle upward to the left - under the Red signal of the main aspect which will not go out.
So - that's what can be done with the signals themselves...
The next thing is to control the trains approach by when the signal is cleared.
For mechanical signals (semaphores) this means that the watching Signalman will keep the signal into the loop at Stop/Danger until he sees the approaching train is running under control and able to stop dead at the signal - or that the train has actually stopped dead at the signal. Only then will he clear the route into the loop - for which he will usually have already Reversed the points. This control of the train before entering the loop should normally mean that the train doesn't slide out of the exit end.
With Colour Light Signals (and with Motorised semaphores - although it doesn't tend to be done with them - that I know of) it is possible to achieve the same control by slowing to a Red signal by arranging the signal to be "Approach Controlled". When that is done the signal into the divergence will not clear unless the train is detected to either be travelling sufficiently slowly or at a stand.
That was good wasn't it?
Meanwhile - back at aversion therapy...
It is in the interest of train crews, particularly those at the front, to not crash into things... So they will manage the speed of their train appropriately so that there is a minimal chance of it over-running the signal protecting the exit from the loop.
When a train is an unfitted or part fitted goods (i.e. it has no continuous automatic brake or is only provided with continuous automatic brake at the front end - a "fitted head" - the crew will be extra wary about running toward any signal that is against them. Working together the footplate crew and guard will slow the train as they approach the Red signal. Doing this they will keep all the couplings stretched out as much as possible... So - they are likely to slow right down and then draw toward the signal - because the slowing part will tend to let the slack in the couplings "run in" so that they drop slack - and then the action of drawing forward (gently) will draw them all up and tight again. (In fact the guard will apply his brakevan screw brake as the train slows so that the van at the back will hold back the train and tend to keep the couplings from slacking...
All that lot means that when the signal into the loop is against a train - and even when it is cleared - the crew will be working to have and keep their train under control so that, having slowed down, they will then draw forward to where they want to stop.
I am now going to have a coffee!
