The "pit" and "plow" mentioned had to do with electrical pickup. The dark slot between the rails (especially visible in the foreground) led to a channel with a third rail. The plow was a contact which was lowered into the slot for the system used within the city limits; outside D.C. they used overhead lines, which were cheaper and more easily maintained. At the boundary the plow had to be put in or pulled out at the pit, and the trolley pole raised or lowered. Obviously if the plow got caught it could derail the trolley, which is why the motorman was pointing that out.

This line survived to the end and can be seen on this map of the lines in 1958. The accident site is at the lower left corner of that little rectangle just SE of the Eckington car barn (see the red triangle at the upper right of the map). You can see the pit marked in-between. The reason for the rectangle is that for whatever reason the inbound and outbound lines diverged for a block at this point. The car actually was running on the near track -- he must really have been flying!