If water is trapped between two planes on a train, and the train decelerates at 4ms^-2. What angle does the surface of the water make with the horizontal?

Draw a diagram of forces on a single particle at surface of water.

Water is stationary in reference to train, thus when including decelerating force, resulting force is zero.

Balance horrizontal and vertical components:

ma = Rsin(theta) (Horrizontal) 

mg = Rcos(theta) (Vertical)

Divide on by the other:

tan(theta) = a/g

(a = 4 ms^-2)

theta = 22.18