If 33 people choose to double it, the entire world's population will be laying on that track, and there'll be nobody to pull the lever. In that case, assuming escape is impossible, we'll have ended the human race.
Well that is a good point, but this assumes that the lever were so organized that everyone would stand as close to each other as possible. If you would distribute levers and humans with a certain amount of distance the effect would be intensely weakened.
I wonder what the optimal trolley track configuration would be to allow everyone to attend their lever while suffering as little disruption to their life as possible, and how much it would cost to build that, and how long it would take for the trolley to make the entire journey from start to finish...