It's not impossible, but: 1. It's expensive, and that cost doesn't work out when you have petroleum available as a fuel and can just carry your own energy sup

It's not impossible, but: 1. It's expensive, and that cost doesn't work out when you have petroleum available as a fuel and can just carry your own energy supply around easily in a small tank under the passenger compartment. It probably still doesn't work out when you have coal, or coal-derived diesel, or vegetable-derived diesel, or "good enough" batteries, available, because those systems are simpler and don't have the funding problems (most of the cost is shifted to the user of the vehicle, which prevents the politically crippling cost of a public system). 2. When you finally designed a system, and standardized the vehicle heights and worked out all the other issues, I suspect you'd have reinvented electric trains. (Or some sort of essentially train-like system, or a system that's not advantageous enough over trains to warrant building it.) Basically it's an idea that's pinched between existing technologies that are good enough in their current forms, and has an extremely high implementation cost. The risk-factored ROI just doesn't appear to be there.