I believe that after SMART gets going, and regular traffic begins to clean the rails and wheels, they will have far fewer problems. But "far fewer" shunting problems ain't good enough. Having "NO" shunting problems is essential to public safety.

I have heard this notion of certain cars being too light to reliably shunt, for over 40 years, but I remain skeptical. In my time, I have performed hundreds of "lab tests" on various types of track circuits, using all sorts of on-rail equipment, including empty flatcars with tare weight as low as 35,000. Even those "lightweight" flats provided good response curves on our chart recorders.

It was mostly vehicles with non-solid axles that gave most of the problems. A lot of DMUs and LRVs do have these types of axles, and so can be expected to have shunting problems.

Also, exclusive use of cylindrical wheels rather than conical, showed problems on the charts. Because of minimal hunting they do not clean the rail or wheel very well, leaving only a narrow clean band on the wheel and on the rail, that was free of rust. When lateral motion does occur, it then causes shunting disconnects due to intermittently riding on rust.

Then there is the problem of how a solid axle is fabricated. At best, two solid wheels are hydraulically pressed onto a solid axle of the same type of metal, without non-conductive grease to form a dielectric metal junction.

At worst (BART for example), there is a ferrous wheel tread pressed onto non-ferrous wheel hub, which is then pressed onto a ferrous axle; with the same construct in reverse at the other wheel. Add in the rail to wheel interface, there are, count 'em, six (6) galvanic metal joints, all of which contribute to difficulty shunting the track. Add to that even more, the use on non-conducting grease during the pressing process --- And you have a recipe for are sorts of shunting problems.

Serious shunting problems are also presented by non-metalic hubs in many LRVs; requiring copper jumpers (not very reliable) between wheel treads and whatever axles there are. Worse yet, non-continuous axles must be jumped by some sort of slipping contactor, (like a motor brush) - which are notoriously difficult to maintain to the necessary parameters needed to adequately shunt the track.

So how does this happen? Mostly engineering types that are inexperienced in the peculiarities and subtleties of railroad physics, that get into positions of authority at public agencies and at their contractor / vendors. In fact, BART's original prime engineering contractor - PBTB (Bechtel), did specify that the track circuits shall be able detect a .060 ohm shunt. This the originally supplied Westinghouse track circuits did manage - sort of. There was often trouble meeting this spec near the source end of the block. But the real problem was that Bechtel failed to provide any spec whatever for how well the cars should be able to shunt. They were then in fact built unable to provide any where near a .060 shunt that the track circuit was capable of. It is not surprising that they had train detection problems.

It should be noted that in response to this incredibly stupid reality, BART's newer track circuits installed in more recent years are much more capable, and can reliably detect a 1/4 ohm shunt. Given the similar problems light rail cars and some DMUs are likely to have in presenting an adequate shunt; it would well behoove all transit agencies using these flaky cars, to use BART's tougher track circuit specification. Moreover, it is long past time that systems engineers consulting for these agencies start doing their jobs and foresee these specification mismatches, correcting them before the concrete gets poured.