South San Francisco's Caltrain station is one of
the few stations left on the line that still has
single-side boarding. Yes, Rule 6.30 is in effect
at that place, which ( think) mandates that no more than
one train is allowed to be in such a station at a time.

I believe that the biggest reason that the South
SF station still has single-side boarding is because
of the yard just to the east of the Caltrain tracks.
I also remember reading that Caltrain had plans to
move the South SF station to another location, which
would have been designed with outside boarding and
pedestrian gates -- as is the case at many Caltrain
stations now,