Consider motion of an acceleration.

Let a box is lying at bottom of compartment of train.

– If box is lying stationary with compartment of train with motion of train, box also will get accelerated.

When train accelerate, friction force acting in horizontal direction will make box to move in direction of train.

If there was no friction, bottom of train would have moved further, box will try to retain its position due to inertia and would collide with rear part of compartment. But this do not happen in day to day life.

Thus, it can be said that static friction force opposes impending motion.

– This impending motion is opposed by static friction force ( $\mathrm{f}_{\mathrm{s}}$ ). This friction force provide equal acceleration in opposite direction of acceleration of train and keep box stationary.