Even a small pressure differential might be a huge boost. At least from what I've seen in my basic physics lab, you don't have to get anywhere near a vacuum or

Even a small pressure differential might be a huge boost. At least from what I've seen in my basic physics lab, you don't have to get anywhere near a vacuum or even what most people would call "low pressure" for it to make a significant difference. Just running a tube between two cities of different elevation might allow for zero energy transport of small pods.