Do nothing, since an infinite number of people implies an inconceivable population overgrowth, so the best possible good for humanity is to cull the population.

Heck, you could probably go out and genocide the rest of the population that isn't tied to the track and still not suffer any real loss. Then, you face the last true enemy: the bloodsoaked beast responsible for the deaths of untold billions- yourself.

Once you've slain that last creature, all of humanity that still remains will be those tied to the railroad track. The only living people will spend their entire lives knowing nothing but the track and the trolley, and the imposing fear that one day, they, too, shall be crushed under its wheels like those before them.

The only life remaining for the human race is now one of terror and eventual slaughter. There are no good outcomes to this conundrum. There are only the uncaring wheels of the trolley.