The angle of the pendulum with the vertical is 
Explanation:
As the train decelerates, the bob of the pendulum will feel a force given by

where
m = 0.55 kg is the mass of the bob
is the magnitude of the acceleration
In the horizontal direction.
The pendulum will be inclined at an angle
from the vertical, so it will be in equilibrium, and therefore the horizontal component of the tension in the string must be equal to the net force F of the previous equation:
(1)
where T is the tension in the string.
We also know that the bob is in equilibrium along the vertical direction: so the vertical component of the tension must be equal to the weight of the bob,
(2)
where
is the acceleration of gravity.
Dividing eq.(1) by eq(2), we get:

And therefore, we find the angle:

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