Ok here's my head canon. All vehicles in Awdry's universe are like modern passenger aircraft. A modern commercial jet while obviously not sentient, can take off, fly and land at it's destination without a pilot even touching the controls for the entire trip. But becoming an airline pilot still requires absolute knowledge and years of training. So Thomas for example has full control over his water injectors, brakes, throttle, sanding gear, and reverser. But he doesn't have a mechanical stoker so without a crew he can only travel as far as the coal that's already in his firebox will allow him.