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You should have seen 4-SUBs at Tulse Hill. On the London Bridge side
there was a porter's crossing about half way along the platforms and I
used to watch trains start off & groan out of the station with
satisfying fireworks every time a shoe reached the gap. In daylight you
could see showers of orange incandescent fragments of something (shoe?
rail?). At night the blue flash would be too bright for these the be
easily seen. (I can still hear what the French call le chant des
moteurs...)

One evening in 1964 I was at the Victoria end of the island platform at
Streatham Common, and I could see (and hear!) some kind of PUL/PAN
formation approaching at speed on the down fast. It was belting along.
It was always satisfactory to be on the platform when they rushed
through. As the train came into view, I could not help noticing (it was
dark) that each shoe on the juice rail side was making its own little
shower of sparks. They were yellow-orange rather than blue, and I
wondered if they were caused by brief loss of contact due to the
swaying bouncy ride these sets had especially at speed.

I've always wondered: with third-rail EMUs such as VEPs, are all the shoes
connected together along a common bus that runs the full length of the
train, or does each powered carriage have its own shoe(s) which are separate
from other power carriages? In other words, is it possible for one power car
to lose its supply (if all its shoes hit a gap) while another power car is
fully powered? Or does a gapped shoe simply mean that less current can flow
equally through every motor on the train? Given that trains usually have
shoes opposite each other on either side of a train, and the third rail is
usually overlapped where it changes from one side of the track to the other,
why do you get such magnificent firework displays when a shoe hits a gap?