My prime suspect would be all those "new" design substations put in from the nineties on, including the ones used on the extension from Concord to Pittsburg. Within the first couple on years or so of the first of these newer installations, several major substation electrical fires were experienced due to overheating in their main transformers.

AS one electrical engineer specializing in traction power at BART told me at the time, this overheating was caused by inadequate iron in the core of the transformers. This also produced a secondary problem, that of increased harmonic electrical noise jamming certain types of BART track circuits at certain locations.

He also indicated that certain modifications were being considered at the time to improve cooling of the transformers. But that the actual origins of the problem, inadequate iron cores, would not be changed, as transformers with adequate iron cores would be much more costly. He has long since retired, but he was the only one there at the time honest enough to admit the real cause of the problems.

How does this translate into today's problems?

Even if PG&E provided a clean balanced three phase sine wave (it often fails to do so), any transformer that does not have an adequate iron core for the amount of current supplied by it, will become magnetically saturated at some point in each cycle. And when saturation occurs, the transformer can no longer provide proper impedance to current flow in the saturated portion of the AC sine curve (near the current peaks). In other words, it very briefly shorts out, twice per cycle; resulting in heavy current spikes in the primary coil, causing excessive heat generation. This capping of the sine wave also causes severe harmonic distortion and noise that is then impressed onto the third rail, and hence into the track circuits.

Thyristors (sometimes called SCRs) are very sensitive to spurious noise pulses from any source, even ringing in a capacitor or choke coil could do it, as will any amount of harmonic noise. Once triggered, they remain conductive until the current goes to zero.

It is a bit like the premature firing of a spark plug, setting off a too early flame front in the cylinder, called knocking or pinging. Enough of this eventually damages the pistons. I would venture a guess that thyristors get damaged over time too, becoming more sensitive to this abuse as time goes on.

A 2nd possibility is that when they improved the cooling of the substation transformers, they did only enough to stop the fires, and no more; cooling just enough to bring temperatures down below the combustion point, but still above the temperature rating of the insulated wire used to wind the transformer. This will eventually over time, cause that insulation to break down or weaken - possibly causing brief intermittent shorts in the coil windings. If that results in the net number of turns in the primary winding being reduced, a HUGE high energy voltage surge of many thousands of volts could result, easily damaging anything down line, such a train's traction power control circuits.

Obviously, transformers afflicted with this malady are in eminent danger of a sudden catastrophic failure. And likely have been shut down, leaving a serious power shortage on the outer C-Line. Also consider that with so many cars sidelined, there may not be enough cars left to provide extended service, even if the electrical power was still there. If the problem is indeed this serious; resolving this is likely not going to be quick or easy, nor will it be inexpensive.