Constant points failures on WAML

[saismee]

I've been noticing frequent (1-2 times per week) that there is disruption on the West Anglia Main Line. It is either reported as a signalling fault or a points failure (or both like today...). Is this average or is this a worsening problem?

 

[Bald Rick]

I've been noticing frequent (1-2 times per week) that there is disruption on the West Anglia Main Line. It is either reported as a signalling fault or a points failure (or both like today...). Is this average or is this a worsening problem?

Its classic for this time of year and these weather conditions. Warm daytime, cold at night. The metal out there is going through some big thermal expansion cycles, and this will particularly affect points, track circuits ( the insulated rail joints) and some types of lineside equipment. It’s the same in many other places. 4 on the MML south of Bedford today alone.

 

[Magdalia]

A recent weather report mentioned a place in Scotland where the temperature rose from a night minimum of -3C to a day maximum of 23C, a difference of 26C.

Here in the Fens we have has a few days recently with night minima around 0C and day maxima at 15-17C. Daytime temperatures at rails exposed to direct sun will be quite a bit higher than that.

 

[jammmmmsssssss]

Yesterday evening, I believe there were four separate signalling problems between Tottenham Hale and Liverpool Street

 

[Annetts key]

Its classic for this time of year and these weather conditions. Warm daytime, cold at night. The metal out there is going through some big thermal expansion cycles, and this will particularly affect points, track circuits ( the insulated rail joints) and some types of lineside equipment. It’s the same in many other places. 4 on the MML south of Bedford today alone.

With point operating equipment, the temperature difference between cold and hot can result in an adjustment that may have been slightly marginal (but within specification when cold) going out of specification when hot. Hence resulting in a failure. Powered points are less effected compared to mechanical points (operated from a lever in a signal box or from a ground frame) that have long rodding runs. But all types may be affected to some degree. As the metal parts get hot, they are affected by heat expansion.

IRJs (insulated rail joints) are the biggest problem at this time of the year. These are joints between different sections of rails that are provided to separate different track circuit sections. To a lay person, they may look like any other rail joint that uses fishplates.

When it's cold, the steel rails will be in tension. On a hot sunny day, the rails will be in compression. Coupled with the vibration and weight of trains passing over, this causes the weaker plastic (compared to the steel rails and bolts) insulation to wear. Once the insulation is bypassed, the resulting short circuit means that the track circuit thinks it's detecting a train continuously in that section.

There are multiple failure modes. The most obvious (if you are stood on the track looking at it) is a failure of the piece of insulation between the rails, called the T-piece. It either breaks in the middle and part or all of falls out. The top of the rail may burr over bypassing the T-piece insulation. The top or the bottom of the T-piece becomes so compressed due to the pressure from the expanded steel rails that the rails squeeze the plastic until it is so thin that the steel rails can push through and touch, bypassing the insulation.

Much less obvious, and impossible to see (until the joint is taken apart), are failures of the insulation around the bolts or the bolt holes. Rather than the rails touching directly, instead the insulation between the rails and one or more of the the bolts fails, or between the bolts and the metal fishplates. Either way, the insulation failures result in a short circuit.

Most electrical and electronic signalling equipment is not affected by normal temperature changes (including these spring time cycles between cold and hot, all this being a consideration in the design). However, some types of electronic equipment may have components that are starting to degrade. The equipment still works fine when in the normal moderate "Goldilocks" temperature range, but will fail when it gets very cold or very hot.

You may say that it's not been that hot. Although the air temperature may not be outstanding, anything in full sunlight for an extended amount of time with no breeze may well be getting very hot.

That includes rust brown coloured steel rails, point rodding and equipment enclosures / location cases/cabinets/cupboards. The latter may get so hot that you could almost fry an egg on them.

By the time that summer actually happens, the failure rate drops because most of the items that were suffering from wear or degradation have already failed and been replaced. And the night time temperatures are not dropping as low as they are now, so it's easier to keep the adjustments for points in their normal range.

 

[Bald Rick]

With point operating equipment, the temperature difference between cold and hot can result in an adjustment that may have been slightly marginal (but within specification when cold) going out of specification when hot. Hence resulting in a failure. Powered points are less effected compared to mechanical points (operated from a lever in a signal box or from a ground frame) that have long rodding runs. But all types may be affected to some degree. As the metal parts get hot, they are affected by heat expansion.

IRJs (insulated rail joints) are the biggest problem at this time of the year. These are joints between different sections of rails that are provided to separate different track circuit sections. To a lay person, they may look like any other rail joint that uses fishplates.

When it's cold, the steel rails will be in tension. On a hot sunny day, the rails will be in compression. Coupled with the vibration and weight of trains passing over, this causes the weaker plastic (compared to the steel rails and bolts) insulation to wear. Once the insulation is bypassed, the resulting short circuit means that the track circuit thinks it's detecting a train continuously in that section.

There are multiple failure modes. The most obvious (if you are stood on the track looking at it) is a failure of the piece of insulation between the rails, called the T-piece. It either breaks in the middle and part or all of falls out. The top of the rail may burr over bypassing the T-piece insulation. The top or the bottom of the T-piece becomes so compressed due to the pressure from the expanded steel rails that the rails squeeze the plastic until it is so thin that the steel rails can push through and touch, bypassing the insulation.

Much less obvious, and impossible to see (until the joint is taken apart), are failures of the insulation around the bolts or the bolt holes. Rather than the rails touching directly, instead the insulation between the rails and one or more of the the bolts fails, or between the bolts and the metal fishplates. Either way, the insulation failures result in a short circuit.

Most electrical and electronic signalling equipment is not affected by normal temperature changes (including these spring time cycles between cold and hot, all this being a consideration in the design). However, some types of electronic equipment may have components that are starting to degrade. The equipment still works fine when in the normal moderate "Goldilocks" temperature range, but will fail when it gets very cold or very hot.

You may say that it's not been that hot. Although the air temperature may not be outstanding, anything in full sunlight for an extended amount of time with no breeze may well be getting very hot.

That includes rust brown coloured steel rails, point rodding and equipment enclosures / location cases/cabinets/cupboards. The latter may get so hot that you could almost fry an egg on them.

By the time that summer actually happens, the failure rate drops because most of the items that were suffering from wear or degradation have already failed and been replaced. And the night time temperatures are not dropping as low as they are now, so it's easier to keep the adjustments for points in their normal range.

A perfect explanation of the situation. I suggest you keep that safe and be ready to post it next year. In fact I might pinch it for some internal reporting