Why is it easier to keep your position steady once the train is moving at a constant speed and direction?

Since the train is moving at constant speed and direction, the velocity of the train is constant. According to Newton's First Law of Motion, when an object is moving at constant speed in a straight line (aka constant velocity), the resultant force experienced by the train is 0N. Since you're on the train, the resultant force experienced by you is 0N, thus, you do not have to exert a force to remain balanced.