There has been plenty of discussion about the scope and range of FB (of which FB XL was borne). I’ll look for the exact threads and post them here if I find them, but the take away is that your run up against structural integrity and precision constraints, especially when increasing along the y-axis. A point will be reached when the combined weight of the gantry (cables, structural materials, water in the tubing, etc) will compromise the current design, causing stability and movement issues.
I do like your idea of of an electrified third rail. You enter a new realm in which there’s a tradespace b/w power supplied, the efficiency of the operation, the electric shock of people and animals, and weight of the infrastructure. At the power levels needed to run a heavy rail commuter rail, it is expensive to shield the cables. It’s also highly inefficient and practical to ‘bring the cable with you’ given the distances involved. It’s also much more economical to put dielectric spacers after 10 feet or so since the third rail is somewhat self supporting (and does not need to support the weight of the train). Finally, the protection mechanism is pretty simple: curved aluminum.
In terms of the FB, you’re hopefully not at the power levels of a heavy-rail commuter train However, with the third rail, you could gain enough power to say run a tiller or a dump shovel of some kind. This would drive up the need for heavier materials and more powerful drive motors, and so on. From a safety stand point, you still would have to protect that third rail from accidental contact. Also, you’d have to insulated the live rail from the non-live rails. That usually means some kind of dielectric material spaced as far apart as the structural integrity of the third rail will allow. If you support it and protect it completely along its length, then you just made a sheathed conductor, or a wired cable .
My take away is that the design would have to radically change in order to necessitate a third rail. Please note, this is a conversation that I had with myself while trying to wake up with my first cup of coffee, so please grant me some grace if some of the ideas are not fully baked.