Disclaimer: I'm starting a job at Hyperloop soon, but haven't actually been told anything about the hardware yet (I'm joining as a software dev), so this is all

Disclaimer: I'm starting a job at Hyperloop soon, but haven't actually been told anything about the hardware yet (I'm joining as a software dev), so this is all not-NDA-violating speculation: I believe the cars are also pressure vessels and life support systems. My student team looked into this side of the problem last year, and making the entire ~40 ft long Hyperloop pod a pressure vessel is really tricky. The two competing forces are the obvious premium on extra mass, and the need for a large opening to load cargo into the pods. Dealing with all the "human compatibility" problems at the scale of the cars lets you use much smaller pressure vessels (lighter), and makes loading unpressurized cargo drop dead easy. The other big benefit is that loading the Hyperloop pods is really difficult. You'd ideally like to be launching a pod into the tube every 30 seconds or less, to get the best return on your infrastructure investment. Achieving that by walking people right up to the 40 ft vehicles and having them all file in like a commercial flight is just about impossible. With mobile sub-pods you can have a far smaller station, and just load sub-pods as they arrive into the next departing pod.