Overlap (OVL) – WikiRail

In the context of ERTMS (European Rail Traffic Management System), overlap, exit zone, is a safety zone provided to provide an additional margin of safety beyonda train’s stopping point (EOA, End of movement Authority) . It is especiallyimportant in Level 2 and 3 operations , where train control relies on continuouscommunication and supervision by RBC (Radio Block Center).

The Overlap is timed in accordance with the release times of the exit zone of the set Itinerary. Entities included in that overlap zone, should be kept blocked by the Itinerary, according to the IXL “logic”.

An overlap refers to a predefined section of track beyond a train’s MA end point (EOA) or intended stopping point. This section acts as a safety buffer to account for:

• Possible overshoot by the train of the EOA;

• Errors in braking performance;

• Unforeseen accidental conditions, such as slippery tracks.

Main characteristics of the overlap:

1. Purpose:

To prevent collisions or derailments in the event that a train exceedsthe predicted EOA stopping point;
Provide increased safety by providing a controlled area beyond the EOA where no other trains are allowed.

2. Length of overlap:

▪ The length of the overlap is determined by various factors, such as:

Train braking performance;
Allowable speed;
Track slope.

▪It is usually a fixed value defined by infrastructure managers and safetystandards.

3. Dynamic Overlap (eventually):

In some ERTMS implementations, the length of the overlap can be dynamically adjusted according to operating conditions, such as trainspeed, weight and braking capacity.
Level 1: The overlap is managed through fixed signaling, with overlapstypically defined within block sections.
Level 2 and 3: the overlap is managed digitally and communicated to the train via the Radio Block Centre (RBC). The EOA and iscontinuously updated based on real-time train positioning and movement clearance.

How the overlay works:

4. Overlay setting:

The RBC or IXL system calculates the EOA for the train, including the overlap section.
The train is instructed to stop before the EOA, but the system ensuresthat the overlap is clear of other trains.

5. Monitoring:

The on-board European Vital Computer (EVC) monitors the train’sposition relative to the EOA and the overlap.
If the train passes the stopping point, the overlap provides an additionalsafety margin.

6. Fault Scenarios:

If the train unintentionally enters the overlap, the system can activateemergency braking or alert traffic controllers for immediate action.
In degraded modes, the overlap can play a crucial role in accident prevention by allowing limited movement under controlled conditions.

Example of overlap in action:

• A train receives permission to stop at a station platform;

• Beyond the stopping point, there is an overlap section (as much as 200 meters);

• If the train passes the stopping point due to delayed braking, it enters the overlap, which provides additional space to stop safely without entering the next block section or without coming into conflict with another train.

Importance of the overlap:

• Increases safety by accounting for potential errors or variations in trainoperation;

• Provides operational flexibility, especially in high-speed or high-traffic densitylines;

• Allows for controlled handling of unexpected overruns without immediatelydisrupting overall traffic.

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