Complex sentence: The racetrack was also the scene of the 1973 Summer Jam at Watkins Glen rock festival attended by an estimated 600,000 people, nearly twice the crowd at 1969's historic Woodstock Festival and a world record for the largest number of people at a pop music festival.
Split into simpler sentences: The racetrack was also the scene of the 1973 Summer Jam at Watkins Glen rock festival attended by an estimated 600,000 people. At the time, this set the world record for the largest number of people at a pop music festival.