Spiritual advancement. The more efficient the mode, the more effective and rapid is the advancement. The rocket would be the most rapid and the highest traveling. Crawling would be among the least effective. A train is forceful and direct, but is confined by narrow tracks. A car is fairly efficient and maneuverable. The airplane is more efficient than the car or train and rises higher than any surface mode of transport. Roller skates are more effective (faster) than walking, but require a smoother surface and more effort; etc.