That is absolutely correct. Lower frequency AC track circuits and coded DC track circuits bounded with insulated joints (IJs), are indeed much more robust. But the mid range audio frequency track circuits used at several rapid transit systems since the late 60s, have the advantage that they do not need insulated joints, which were a huge curse to the industry until fairly recently.

Most railroads, with their average track circuit length being between 5000 to 10000 feet, would likely just put up with IJs. But rapid transit blocks are commonly as short as 400 feet (for high density traffic), producing a huge maintenance headache. But of course, now that bonded insulated joints have solved the maintenance problems associated with IJs; the benefits of the AF (audio frequency) track circuit may well now be outweighed by their disadvantages.

But even with 100hz circuits, and especially with 60hz and coded DC circuits, great care is still needed to prevent "foreign currents" from interfering. And as we see from the recent WMATA accident, circuits must be designed to fault to a safe (occupied) state, no matter what kind of interference is occurring. Coded DC, and low freq. 100-250hz track circuits do much better at that than AF circuit can ever hope for.

Indeed, the NTSB just days ago issued an unprecedented "urgent warning" that false signals induced into AF track circuits could be accepted as the correct signal. This is now their prime suspect cause of the recent WMATA accident. Transit agencies all over the country are now scrambling to assure that they are not also experiencing these types of interferences. But to be clear - WMATA was experiencing a very different sort of interference that what we have been describing here.

At WMATA, the interference mimicked the intended signal, and thus was accepted by the circuit, falsely clearing the block even though the signal did not come from the correct source and pathway.

Because audio frequencies can easily radiate all over the place, and sneak around on paths that no man could scarcely even dream of, AF tracks can easily interfere with each other; especially when concentrated en mass in just a few centrally located rooms, as rapid transit agencies tend to do. Railroads rarely do this, instead distributing their equipment along the wayside. Where it belongs, I might add!

While any type of interference should cause trains to stop; extreme care must always be taken to make sure that, that IS what really will happen. It didn't happen at WMATA. It all goes to show, as I have stated many times before on this forum - that it simply will not do, to merely replace human error with a machine's error.

The machine must be designed, in so far as humanity is able, to ALWAYS revert to a more restrictive state in the event of any fault whatsoever. And that, is a damn tough standard to meet. But any new technology, even for PTS, had damn well better meet it. Or we're just spinning our wheels - towards another machine caused accident! I suspect (hope) that the days of transit's preference for AF track circuits, all crammed up in the same room, may indeed be numbered. And let's also hope that, transit's preference for F.R.A. non-compliant cars, is also done away with.

OPB