I, also, have long been fascinated with the electro-pneumatic train brake system. Was it originally tried in the 1930's on passenger trains?

As it would require a trainline electrical circuit, it would not be easy to phase it into the existing car fleet, which is why it would start out with unit trains and other dedicated- service train sets. I don't think it would be much of a target for copper thieves as it would be small-gauge wire. It would only need to carry signals, not power.

I assume that with electro-pneumatic brakes, the trainline air pipe is kept fully charged at all times and the electrically controlled triple-valve applies the train brakes with air from the individual air reservoirs on each car. The response would be very very fast and uniform throught the length of the train. If the electrical trainline circuit fails, reduction of air in the trainline pipe applies the brakes in the conventional manner.

Question: I have seen unit-trains with a second, smaller-size trainline pipe. What does it do and how does this system work?