Right now, it's all still early stages — but public patent filings can offer us tantalising glimpses of what Amazon's engineers are thinking about and experimenting as they develop the tech.

For example, a key problem facing any drone deliveries is batteries and maintenance. When your drones are in the shop getting fixed, they're not helping you make any money — so how do you keep them charged and in the air for as long as possible?

One possible answer: An ambitious fleet of mobile maintenance facilities based on trains, in vehicles, and on boats.

In a patent filing published by the United States Patent and Trademark Office earlier this week, Amazon reveals it is thinking about exactly this (emphasis ours):

"Intermodal vehicles may be loaded with items and an aerial vehicle, and directed to travel to areas where demand for the items is known or anticipated.