Level Crossings: A Guide on How Trains Activate Crossings
After talking to someone recently about level crossings and what makes them so dangerous, I found that it can be tough to understand how a level crossing works and what happens when they don't work. So I put together this guide based on my experience and understanding of level crossings to help anyone answer these questions.
The sequence of how trains activate and pass through a level crossing:
- Approaching trains are detected by sensors, such as track circuits or axle counters
- Barriers and flashing lights activate with sufficient time for vehicles and pedestrians to clear the intersection
- Once all parts of the train have cleared the crossing, the barriers lift and lights stop flashing
A full sequence of level crossing operation is available as the first reference at the end of this article1.
I've also looked at why crossings can stay closed for a long time even if there is no train there.
Approaching trains are detected by track circuits or axle counters
Sensors used to detect train position identify when a train is approaching a level crossing. These sensors can include traditional track circuits, axle counters, or more advanced systems such as highway crossing predictors.
Trains trigger the sensors when the train is within a certain distance of the level crossing. This certain distance represents sufficient warning time for the crossing bells and barriers to activate to warn pedestrians and vehicles to leave the crossing.
Once a train has passed over the sensor point, the crossing bells and barriers will remain active until the train exits the crossing.
Adjusting for train speed
A fast train approaching a level crossing requires more distance to achieve the warning time. If a train is moving slowly when it passes the strike-in point, then the crossing will stay closed for a long time.
Newer sensors are able to calculate the train speed, called grade crossing predictors or highway crossing predictors2.
By calculating the train's speed, the highway crossing predictor can reduce the amount of time a level crossing spends closed. This can make it less frustrating for people waiting at the level crossing.
Barriers and flashing lights activate
Barriers and flashing lights will activate soon after a train has passed the strike-in point.
All the level crossings I've worked with follow the same sequence for the activation of barriers and flashing lights:
- Train passes strike-in point
- Bells start ringing; lights start flashing
- Barriers start to lower after some time has been allowed for vehicles and pedestrians to exit the crossing
I've also seen level crossings at road intersections where an approaching train will trigger the road traffic lights in sequence to allow vehicles to clear the crossing.
Crossing deactivates once the last part of the train has gone through
Once the last part of the train has gone through, the barriers will raise, the bell will stop, and the lights will stop flashing.
As long as there is any part of the train still in the road part of the crossing, the barriers will stay closed.
This means that if a train can't completely leave the crossing, the crossing can't open again for road use.
Why crossings stay closed for so long
A crossing can stay closed for a long time due to any of the following reasons:
- The train may be prevented from clearing the crossing
- The train is stuck on the approach to the crossing
- The crossing is broken
Train is prevented from clearing the crossing
A crossing will stay closed if any part of the train is still in any part of the crossing. This means a train has to completely leave the crossing before the barriers will open up to road users.
I've found trains are often prevented from leaving the crossing due to3:
- No route available for the train on the other side of the crossing
- Track ahead of the train is not clear
No route available means that the path for the train to travel on may not be available after the level crossing. If the train was destined to go into a siding, for example, and the track had not yet been set such that the train could proceed into that siding, then the train will continue to occupy the level crossing.
If the track ahead of the train is not clear then the train cannot safely exit the level crossing; this is part of the system that prevents trains colliding.
Train is stuck on the approach to the crossing
If the train stops in the approach section of a crossing for any reason, the crossing will continue to be active.
I've found trains typically stop on the approach section due to one of these reasons:
- A station platform is located in the approach section and the train has stopped at it
- The train is prevented from proceeding as there may be another train ahead
In these scenarios, the level crossing still sees a train "on approach" and will continue to ring the bells and allow the barriers to stay closed as the train may come through the crossing soon.
The crossing is broken
If any of the vital components in a level crossing fail to work, chances are the level crossing will activate.
The safest situation for a level crossing to be in is when the lights are flashing and the barriers are lowered. The level crossing is designed such that if any of its components fail, the crossing will move to the safest state (activated).
This is called fail safe design. Some examples of fail safe design features in level crossings include:
- Weighted barriers that are held up by electricity
- Sensors detect the absence of a train
- Relays that use back contacts
I've found the most common causes of level crossing failure are related to the electricity supply and train detection sensors.
Weighted barriers are held up by the electricity supply. This is why you see large blocks attached to counter the arm of the barrier. If the electricity supply fails, the barrier will lower and protect the crossing.
Sensors that detect the absence of trains. If any of these sensors fail it means they have failed to detect the absence of a train. Therefore, it means there may be a train in the vicinity of the crossing and the safest position to be in is to lower and activate the crossing
Relays use back contacts to activate the level crossing if the circuit is not complete. Back contacts of a relay are contacts which connect a circuit if the relay is de-energised. The circuit energising the relay can fail if a cable is broken or if the power supply stops working.
All of these failure scenarios mean that the level crossing can activate without a train being present
Approaching trains typically activate level crossings using sensors. These sensors detect the absence of a train. If these sensors fail to detect that there is no train on approach, such as by electricity failure, a level crossing can activate even when no train is there.
This can be frustrating for road users but I've found this to be one of the requirements of fail-safe design for railways. In some of the research for this article, I did find alternative systems that can provide this fail-safe feature and simultaneously reduce the likelihood of people being stuck at crossings. Check out the Japan Rail source in particular if you're interested.
For further reading, check out some of the references I used in writing this article:
Siemens Grade Crossing Predictor product manual (PDF) - an explanation of how crossing predictors operate. Also available here if Siemens change their website;↩