Question

Why wooden or concrete plankd are kept under the railway line?

Answer 1

Railway tracks are laid on large sized wooden or iron sleepers beacuse weight of train is spread over a large area of the sleepers. Therefore pressure acting on ground under the sleepers is reduced. This prevents the shrinking of the ground under the weight of train.

Answer 2

weight of train is spread over a large area of sleepers. therefore pressure acting on the ground under sleeper is reduced.so wooden or concrete planned are kept under railway line.
hope it helps u