The correct answer is: A. Changing the direction of engine.
A triangle is a railway track layout that allows locomotives to change direction without having to reverse. It is made up of three tracks that form a triangle shape, with the locomotive entering the triangle on one track and exiting on another. The triangle is used in yards and terminals to allow locomotives to move between different tracks without having to back up.
Option B is incorrect because a triangle is not used to transfer wagons to and from parallel tracks without shunting. A shunting locomotive is used to move wagons around a yard or terminal. A triangle is not necessary for this task.
Option C is incorrect because a triangle is not used to separate all the sidings and shunting lines from main lines. A siding is a short track that is used to store or load and unload wagons. A shunting line is a track that is used to move wagons around a yard or terminal. A triangle is not necessary for these tasks.
Option D is incorrect because a triangle is not used to prevent the vehicles from running off the track. A guardrail is used to prevent vehicles from running off the track.