The shoe on top of most pantographs also curves downward slightly at its ends, so the train has to move a considerable distance off the straight-and-narrow before the panto will spring up enough to hook over the line. The overhead line isn't usually rigged to run exactly parallel to the tracks however. If it did so, there would be excessive wear at the central portion of the collector. Instead, the line weaves left and right along its length to even out the wear on the collector shoe. If the train happened to lurch off the rails in the wrong direction when the line was over to one side, it might be easier to hook over it then.
The London Undergound also gives quite an impressive fireworks display at night from the third and fourth conductor rails (630V DC). In this case, there are multiple pick-up points along the train, so if a shoe does lose contact with the rail at some point the spark is just the result of a break in one or several parallel paths.
I don't know if they still do it, but years ago on the above-ground sections of the Underground ice was cleared before the first trains of the morning by shorting the rails together at the end of a section and passing low-voltage high-current through the tracks to warm them sufficiently to melt the ice.
In recent years, our regular train service has become a national joke as to what lame excuse they'll come up with next. One winter we were treated to "It was the wrong sort of snow."
