Monday, June 28, 2021

Safety Message: Short warning times (wheel-rail interface issues)

Level crossings are designed to provide the appropriate warning of an approaching train or trains to motorists, cyclists and pedestrians. This “warning time” is defined as the time between the level crossing activating and when the train first enters the crossing. A short warning time (SWT) is when the time between crossing activation and the first train entering the crossing is less than the minimum design warning time.

These incidents can result in collisions and derailments that can ultimately cause serious injuries and even fatalities. One of several factors that can cause a SWT incident are wheel-rail interface issues and ONRSR’s investigations have shown there are a number of mitigations that can minimise the risks. While not exhaustive, given the range of operational environments within the Australian rail industry, the following causes/contributing factors and controls should be noted.

CAUSES AND CONTRIBUTING FACTORS

Why does wheel or rail contamination cause SWTs?

Electrical train detection equipment (track circuits) work by sending an electrical current through the steel of the railway tracks creating an electrical circuit. The electrical circuit is monitored by electrical equipment (a track relay). When a train passes over the railway tracks, the wheels of the train shorts the electrical circuit. In railway terminology, this ‘short-circuiting’ is known as ‘shunting’ or ‘dropping the track’. If there is wheel or rail contamination, the presence of the train wheels may not shunt the track because the electrical current is not able travel from the rail to the wheels of the train. This poor electrical conductivity can then result in the track relay being unable to adequately detect the presence of the train.

What are common causes of poor electrical conductivity?

• Railhead contamination:
  • includes rust, vegetative matter, weed spray chemicals, insects, grain, grease, oil, mud and excessive sand from rolling stock traction/braking systems.
• Wheel contamination:
  • includes tread corrosion, tread glazing from composition brake blocks and a degraded wheel to rail contact profile. This can result in the wheel not making adequate contact with the rails to allow proper electrical conductivity.
• Type of rolling stock:
  • historically, there have been instances of relatively light mass rolling stock tending to have poor electrical conductivity. Heavier trains with long consists tend to “clean” any light contamination such as rust.

CONTROLS

While not an exhaustive list, ONRSR is aware of the following controls that are available and have been used in railways both in Australia and overseas. rail transport operators (RTO) should note that there are safety and operational benefits and detriments associated with each control. Operators must consider a range of factors, including the likelihood of the hazard and the degree of harm to determine what controls are reasonably practicable to implement – see the ONRSR Guideline – Meaning of duty to ensure safety so far as is reasonably practicable SFAIRP for more information.

Elimination

The following controls are likely to eliminate the risk but this may not be reasonably practicable - see the ONRSR Guideline – Meaning of duty to ensure safety so far as is reasonably practicable SFAIRP for more information):

• Grade separating of the road and level crossing
• Closing or diverting the road, closing or diverting the rail line

Engineering:

The engineering controls railways have employed can be categorised into the following:

Train Detection Systems

Two of the train detection systems can be employed to better detect the presence of trains to reduce the likelihood of SWTs due to wheel-rail interface issues:

• Treadles and/or Axle counters: installing of Treadles and/or axle counter in lieu of track circuits or for assisting train detection where rail contamination or wheel-rail contact is an issue.
• High voltage impulse (HVI) track circuits: HVI track circuits use a high voltage waveform which overcomes film resistance that could result from rust and general contamination. Historically proven to provide improved wheel to rail shunt characteristics.

Trackside Controls

• Remote monitoring: allow for immediate notification of a short warning time and prompt responses to be implemented to ensure network safety and quick rectification.
• Trap track or track sequencing: A design characteristic of the level crossing controls which will keep the level crossing operating despite a loss of track circuit shunt.
• Level crossing predictor settings: ‘motion detect’ or ‘loss of shunt’ settings can be adjusted to ensure level crossing operation in the event of a loss of track shunt or train overspeed.
• Slow to release relays: Designed to remain energised for a predetermined time despite losing voltage over the controlling coils and typically used in track circuit relays to remedy a brief loss of train shunt.

Train-Borne controls

• Track circuit assistors (known as TCA): devices installed on board rolling stock to improve the shunting characteristics of track; improving the operation of a track circuit by increasing the effective track circuit voltage.

Rail-Based Systems

• Installation of Stainless Steel Strips:welded onto the head of the rail within the approach sections of a level crossing to prevent build-up of rust contamination on the rail surface.

Administrative:

Some of the administrative and organisational controls railways can employ to mitigate the risks associated with short warning times at level crossings include:

Track maintenance controls:

• Track patrols to identify rail head contamination, and risk assess and prioritise remedial actions.
• Rail scrubbing and rail milling activities to remove rust, scale and other contaminants from the rail head producing an ideal contact profile between the wheel and rail.
• Protective mats to prevent contamination during adjacent works such as vegetation removal.

Rolling Stock

• Testing existing, new or modified rolling stock to ensure the integrity of on-board /on-train equipment and systems (e.g. traction systems, TCA, ATP equipment, wheel profiles, etc). This can include but is not limited to ensuring that rolling stock:
  • reliably operates the signalling train detection systems;
  • operates in accordance with all technical specifications; and
  • does not cause any adverse impact on train detection systems.
• Ensuring wheels are free of tread corrosion, tread glazing from composition brake blocks and a degraded wheel to rail contact profile.
• Ensuring that sanding systems are functional and are delivering the correct quantities of sand.
• Ensuring de-sanding systems, if fitted, are functional.
• Track Circuit Assistor (TCA) devices, where fitted, are correctly connected and functioning in accordance with all applicable design and technical specifications.

Safeworking

• Book out and disconnection of faulty equipment and install manual safeworking protection: signalling technicians can request to book the affected level crossing out of service, temporarily install road traffic management and rail safe working resources to provide additional protection at the level crossing until the SWT alarm is rectified. SWT alarms should be dealt with promptly and such defects monitored as part of asset management processes. Prompt rectification will reduce need for manual operations.
• Restrictions on road vehicles: this can reduce the risk of a collision by removing heavy and long vehicles (e.g. heavy loads, B- Doubles) from accessing specific level crossings.
• Driver circulars: issuing a formal notice to train drivers as a reminder to note changes to the operation of the rail network as a result of contamination control procedures (such as a CAN warnings or other written advice to drivers). This promotes good practice to ensure consistent understanding and application by drivers.

Training

• Driver re-education programs and auditing: providing positive mentorship while enhancing skills in how to appropriately control a train in specific situations such as speed regulation when approaching a level crossing. This includes emphasis on driving in accordance with prevailing weather and track adhesion conditions.
• Structured, practical, competency-based training of infrastructure workers including:
  • applicable, regular and timely refresher training and mentorship;
  • regular on the job task-based audit/assessment of rail infrastructure workers;
  • ongoing documented practical formal and informal skill and competency training, mentorship and assessment.

This information is provided as guidance only.