10-08-2012, 02:52 PM
Calculating Train Braking Distance
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Introduction
For trains to safely travel on a railway, trains must be
provided with sufficient distance in which to stop.
Allowing too long a distance reduces the capacity of the
line and hence the return on rail infrastructure investment.
Too short a distance and collisions would occur, because
the train would not be able to stop within the available
distance and would therefore occupy a section of track
that could be allocated to another train. Consequently it is
important that the distance be adequate, but not overly so.
Figure 1 shows the relationship between braking distance
and “headway” which is a measure of capacity.
Definitions
ATP – (Automatic Train Protection) is a predictive
enforcement system which continuously monitors the
speed of a train in relation to either a target speed, which
for a Limit of Authority would be zero, or a target
distance, and intervenes such that the train is prevented
from passing a Limit of Authority or exceeding a speed
limit.
Calculating Braking Distance
Influencing Factors
Braking distance depends on:
· the speed of the train when the brakes are applied;
· the deceleration rate available with a full-service
brake application, which varies according to the
coefficient of friction between wheel and rail;
· the delay from when the brakes are commanded by
the train driver to when they are actually become
effective (brake delay time);
· the state of the wear of the brake pads and the air
pressure available in the brake cylinders;
· the geography of the track, in particular the track
gradient the train travels over from when the brakes
are commanded to where the front of the train stops;
Calculation Method
It is impossible to calculate the precise stopping distance
as this distance can vary significantly due to the condition
of the train and the environmental conditions at the time.
To take the conservative approach, i.e. allow for the worst
case conditions would grossly impact on track asset
utilisation. The industry approach is to assume that the
train’s brake system is healthy and that the specified
adhesion for that class of train is available when the
brakes are required to be applied.
Most railways do not take into account the brake delay
time, and further simplify the calculation by using the
average gradient of the track on the approach side of the
Limit of Authority.