Anatomy of an Arc - A Metro Moment

Anatomy of an Arc - A Metro Moment

...or should I have titled it "Denouement of an Arc?"

If you've spent any time watching Subway trains, you're aware of those bright blue-white flashes, or arcs, that occur when pick-up shoes momentarily break contact with the third rail. They can often be accompanied by a loud "Splottt" or "Crackkk." These are often most dramatic when one is standing on an underground station platform and a departing train enters a tunnel, making the flash all the more brilliant in contrast to the surrounding darkness. And certainly they can be seen on above ground stretches of track or those on elevated structures above New York or Chicago streets or London railway embankments, often repeating as each car of the train passes over the same discontinuity in the third rail.

This phenomenon is, of course, not limited to third rail DC operations; one can also see arcs from above, if an electric train's pantograph (Stromabnehmer) momentarily loses contact with the overhead catenary. In either case, it is high voltage electricity, whether 600 or 750 volt DC or 25 KV AC, saying to the pickup shoe or pantograph, "I'm mad as hell at you for breaking up with me. Take that: SPLATT!".

I'm not an expert on arcing, so it might well be that what you see here as a Washington Metro 6000-series train departs Potomac Yard Station in Alexandria, Virginia is the "denouement" of such bright flashes, or perhaps my camera caught a tiny phase of what our eyes generally report to brain as a single, bright "flash."

If you happen to be well versed in DC traction, I'd welcome your comments of, might I say, "enlightenment."

As to "breaks" in the third rail and the visual (and aural) excitement they can provide, often there is nothing wrong or defective going on. As seen in the photo, there must be breaks in the third rail for track switches and other interlocking infrastructure, and the fact is that pick-up shoes of other cars in the train (or even the opposite end of car in question) are indeed in contact with third rail, so there is no impediment to a train's progress and, on all modern equipment, there is not even the flicker of interior lighting.

So, everyone, transit geek or not, here's wishing you a cracking weekend, filled with sparks of excitement.

©2023 Steve Ember