Answer:
The acceleration would be positive, pointing to the right.
Explanation:
The acceleration tells us how velocity changes every second. If the train stops, this means its velocity must go from a non-zero value to zero.
To reduce velocity, acceleration must point to the opposite direction. Since velocity is negative, acceleration must be positive.
As the source approaches you, the sound waves are compressed, so
the pitch of the sound is higher than what the source is actually emitting.
Then, after it passes you and begins moving away, the sound waves
are stretched, so the pitch of the sound is lower than what the source
is actually emitting.
