Imagine you own and operate a train. You worry about brake pads. They are expensive and heavy and slowing the train to a halt wears them out. Moon is your le

Imagine you own and operate a train. You worry about brake pads. They are expensive and heavy and slowing the train to a halt wears them out. Moon is your least favourite station. In order to let passengers off at Moon you have to come to a complete halt. The brakes get hot and wear out quickly. Mars is a much better station to visit, at least for you. It’s surrounded by a mountain of soft airy pillows. You can just push the passengers out the door as you fling past, knowing they’ll hit the pillows and eventually come to a stop. It’s uncomfortable for them but it’s not terribly dangerous and you save a fortune on brake pads. In fact, Moon and Mars are on two different branch lines of the railway. If you can contrive to only operate the branch line that serves Mars station, you don’t even need to bother fitting brakes to your train.