Colour Light Signalling and Operating Critique Required

[Dungrange]

11 hours ago, Dungrange said:

That's why I'm not convinced that the Fouling Point is actually the true physical conflict point between two items of stock but is instead the point at which the clearance between two vehicles is considered sub standard (ie less than around 18"). 

 

31 minutes ago, 5BarVT said:

So what you have described there is CP not FP, but your reasoning is spot on.

 

Okay, but that's the bit that I don't fully understand, because at the point toes, two items of rolling stock sitting on adjacent lines will have a full clearance distance (around 18") between them.  As the first wheel set of one of these items of rolling stock passes onto the end of the switch blade the vehicle will start to deflect towards the other running line and the clearance distance between those vehicles starts to be compromised (ie it starts to become less than the clearance provided during normal running).  What you are saying is that is the Clearance Point.  However, that would imply that it's the Clearance Point that is at the point toes rather than the Fouling Point, which would therefore lie closer to the common crossing (as per my experimental photograph above).

 

However, the Rail Standards and Safety Board (RSSB) standard on Infrastructure Based Train Detection Systems (https://catalogues.rssb.co.uk/rgs/standards/RIS-0728-CCS Iss 1.PDF) sets out the methodology for determining the Fouling Point and the Clearance Point, which as both you and Simon have said earlier is by measuring ~ 2m or ~6' between running edges perpendicular to the line concerned.  The distance specified in the standard is 1970 mm, which is effectively the six-foot plus two rail head widths.  That therefore means that it is the Fouling Point that is located at the point toes and the Clearance Point must be away from the crossover, which aligns with what is shown in Simon's book and his post above and I conclude that is correct.

As such, I don't think trains would ever physically touch at the Fouling Point, even if that is what the name may imply.

 

In the case of a standard crossover, using the definition that the Fouling Point is defined as the point at which the distance between the running edges of rails measured at right angles to the passing train is 1970 mm, the Fouling Point will be the point toes and there will be around 18" between vehicles.  That's definitely not the point at which two vehicles touch, but it does seem to be the definition of the Fouling Point.

 

Furthermore, one of the examples in the RSSB standard above is a perpendicular flat crossing and again in this instance two trains would not physically touch at the Fouling Point.  If I assume that the passing train is on a line cleared to the W12 loading gauge, then it will be capable of carrying containers that are up to 2600 mm wide.  If I subtract the track gauge (1435 mm) and divide by two then the overhang of the passing train from the running edge of the rail would be 583 mm.  If the Fouling Point is measured as 1970 mm from that same running edge, then that implies that there would still be a clearance of 1165 mm between the passing train and a stationary train sitting at the Fouling Point.

 

All I can conclude is that there is an unspecified 'safety margin' built into the Fouling Point calculation, which is in addition to the formal 'safety margin' that forms part of the specified 4.88 m between Fouling Point and Clearance Point. 

[5BarVT]

6 hours ago, Dungrange said:

which is effectively the six-foot plus two rail head widths.

As I was writing I had a niggling doubt that it was the non gauge face that was the measurement.  So 6’ to non gauge face (old style) or 1930mm using gauge face.

Just been back into my 1975 principles (I was young then!) and the aim is stated: SSP 36 says 18” minimum passing clearance at all times.  Achieved by measuring 6’ between outside edges and then 16” further (not the 16’ 6” I said earlier).

Paul.

[Dungrange]

19 minutes ago, 5BarVT said:

As I was writing I had a niggling doubt that it was the non gauge face that was the measurement.  So 6’ to non gauge face (old style) or 1930mm using gauge face.

 

I assume that historically it was easier to measure 6' from the non gauge face, in the same way as the six foot was no doubt set out.  Presumably the change to using the gauge face of the rail came with a move to measuring gauge etc using more specialised equipment and even complete trains.

 

20 minutes ago, 5BarVT said:

Just been back into my 1975 principles (I was young then!) and the aim is stated: SSP 36 says 18” minimum passing clearance at all times.

 

Is that minimum passing clearance at the Clearance Point or the Fouling Point?

 

20 minutes ago, 5BarVT said:

Achieved by measuring 6’ between outside edges and then 16” further (not the 16’ 6” I said earlier).

 

I suspect the difference between 16" and 16' 6" may be a difference in what is being defined.  16" is 406 mm ; 16' 6" is 5.028 m.

 

However, breaking down the 4.88 m quoted earlier as the distance between the Fouling Point and the Clearance Point, this seems to comprise a safety margin of 1.654 m and an overhang of 3.226 m.  However, it also states that a TSI-compliant train (I'm not sure what that means) would have a 4.2 m nose overhang and a safety margin of 680 mm.  

 

I'm therefore going to suggest that the 16" was measured to the physical front of the train, but the 16' 6" was to the track circuit location, which would have to allow for any overhang.  

[TheSignalEngineer]

22 hours ago, 5BarVT said:

Just been back into my 1975 principles (I was young then!) and the aim is stated: SSP 36 says 18” minimum passing clearance at all times.

I remember the 6ft + 16ft rule coming in, it must have been 1973 or thereabouts by the desk I was sat at in those days. 

Two factors came into play, firstly an incident at East Croydon where an EMU had the door handles taken off by a passing train and the advent of long overhangs like the APT. I think that had about 11ft from the wheel to the end. whereas most coaches were about 4ft to 5ft. I remember we had to measure all of the clearance points at Birmingham New Street which was a right PITA to get done safely.

[Grovenor]

Essentially, and as the wording implies, the fouling point is where the kinematic envelopes just touch, the clearance point is the closest point where the required minimum distance between trains exists. In order to provide readily understood rules all the worst case circumstances of overhang, curve radius cant etc. have been allowed for in the principles, and, in addition the clearance point in that rssb document is the position of the first axle, ie where a track circuit joint or axle counter head would be and thus has to include the worst case end overhang as well.

When circumstances make these figures unrealistic, eg long overhangs and sharp curves such as on the DLR it is neccessary to go back to first principles and calculate from the detail track plan and the kinematic envelope just where the FP is, then where the CP is, then allow for the end overhang to locate the axle counter or track circuit joint. Using the network rail measurements did not work.

Rgds

[Dungrange]

Okay, I think I understand now.  In general the kinematic envelopes of two items of rolling stock will not actually touch at the Fouling Point derived from the definition that the Fouling Point is the point at which the distance between the running edges of rails measured at right angles to the passing train is 1970 mm.  However, if the worst case combination of factors that were used to derive that methodology for calculating the Fouling point are met, then they will in fact just touch (or perhaps just about touch).

 

Looking at the examples in the RSSB standard I linked to earlier, the distance between the Fouling Point and any physical contact point would clearly be greatest in the example of a perpendicular crossing.  As the crossing angle becomes more acute the distance between the Fouling Point and the passing train would reduce, which would indicate that the worst case is likely to be a crossover between two parallel tracks.  Where I've been going wrong is that I've been thinking about a vehicle in the 'passing train' that is starting to be deflected towards the other line by the leading wheels at the point toes.  However, that is not the worst case, which is clearly, a short crossover and a very long bogie vehicle with the midpoint of the bogie vehicle being at the Fouling Point to give the maximum centre throw at that location.  If the mainline is curving then there could be end throw on the stationary vehicle sitting at the Fouling Point and I can see how cant may also affect this calculation.

 

I guess in effect there are two different Fouling Points: one derived from first principles, where a gauging engineer actually calculates the intersection of two kinematic envelopes specific to the actual geometry; and a second more conservative location that is essentially always slightly further away, based on a simple and easy to implement way of measuring on site by someone less qualified to actually determine the intersection of kinematic envelopes - ie the definition that it is a point at which the distance between the running edges of rails measured at right angles to the passing train is 1970 mm.

[Grovenor]

Quote

I guess in effect there are two different Fouling Points: one derived from first principles, where a gauging engineer actually calculates the intersection of two kinematic envelopes specific to the actual geometry; and a second more conservative location that is essentially always slightly further away, based on a simple and easy to implement way of measuring on site by someone less qualified to actually determine the intersection of kinematic envelopes - ie the definition that it is a point at which the distance between the running edges of rails measured at right angles to the passing train is 1970 mm.

Yes, that is basically it.