Lapping

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Depending on the motorized vehicle type, there are two ways to control the brake pipe pressure: self-lapping and the older, non-self-lapping.

On the self-lapping systems, the brake pipe pressure is handled automatically. The driver can set the train brake lever to any position in a range. The further it is set in the “apply” direction, the stronger the braking force, and vice versa. The valve automatically handles the dumping or pressurizing of the brake pipe depending on the position of the lever.

On non-self-lapping systems, the process is more hands-on. The lever has a few fixed positions. Two of the positions, called “release” and “service” can be thought of as adding and releasing the brake pipe pressure, respectively. The position in between them, called “lap”, is used to maintain whatever pressure is in the brake pipe. The furthest position, called “emergency”, will release pressure at the fastest possible rate.

Auxiliary reservoirs are being drained when braking. When the brakes are released, it takes some time for auxiliary reservoirs to fill up again. The lap position, however, functions only if the auxiliary reservoirs are full. To stay in full control over braking it is recommended to not release non-self-lapping brakes when knowing that moments later braking may be needed again. Calculated, slow moves are preferable when releasing the non-self-lapping brakes.

With both brake types, the driver can see exactly how much braking force they are applying by looking at the brake pipe and brake cylinder gauge needles.

Both systems are fairly simple to use, although self-lapping may be more intuitive to beginners. In simple terms, with self-lapping the control device sets the desired braking force, leaving it to the automatic valve to do the work, whereas with non-self-lapping the control device directly sets the braking force.

In case a train contains motorized vehicles with both self-lapping and non-self-lapping brake valve types, only the self-lapping ones will be functional when operating the brake. For that purpose, vehicles are also equipped with brake cutout valves, which can be used to isolate certain vehicles from the brake system and resolve this conflict.