A diagram from Amazon's patent filing showing drones being deployed from a maintenance facility on a train. Amazon/USPTO

Amazon is heavily investing in drones, and one day hopes to use the unmanned aerial vehicles (UAVs) to revolutionise deliveries.

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. The intermodal vehicles may be coupled to locomotives, container ships, road tractors or other vehicles, and equipped with systems for loading one or more items onto the aerial vehicle, and for launching or retrieving the aerial vehicle while the intermodal vehicles are in motion. The areas where the demand is known or anticipated may be identified on any basis, including but not limited to past histories of purchases or deliveries to such areas, or events that are scheduled to occur in such areas. Additionally, intermodal vehicles may be loaded with replacement parts and/or inspection equipment, and configured to conduct repairs, servicing operations or inspections on aerial vehicles within the intermodal vehicles, while the intermodal vehicles are in motion."

In plain English? Amazon is exploring the idea of building special facilities that can store, repair, and deploy drones, and pre-emptively moving products and drones to areas of anticipated demand (based on seasonal trends, say, or a special event in the area) before launching them.