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It depends on your definition of "brakes". To my understanding trains have two different types of "brakes". Friction and applied force by reversing the engines.
Therefore, there are two answers depending on the context of the question ....
Given ...
Acceleration =(FINAL VELOCITY - ORGINAL VELOCITY) divided by TIME
We get ...
-2MS^2 = (ZERO - 40/MS^2) / TIME
20 seconds of time before the train reaches zero velocity from the relative point in space for the question.
Therefore the train went the same distance it would accelerate forward at for 20 seconds ...
2/ms^2 for 20 seconds = 40 meters.
However, if the reverse engines are still being applied, which the question appears to be saying ... then
you need to add another 10 Seconds of acceleration and
we add another
20 Meters.
One answer is ...
The train traveled 60 Meters from the relative point in time in which the question was posed.
Quite simply 30 seconds of acceleration at 2/ms^2.
Dad@
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