He was everywhere: he was at the communion rail,
the fifty-yard line
, he played the oboe with the Chamber Music Club, drove the fire truck, served on the school board and rode the 8:03 into New York every morning
Learn English – Meaning of “the fifty-yard line”
idioms
Best Answer
The fifty-yard line is the center line of a football field (American football). To be "at the fifty-yard line" is to be a player in a football team, and actually take part in the game (and not on the sidelines), most likely the beginning, which starts in the center of the field.
EDIT: As StoneyB pointed out, the game starts by a kick-off (unlike the soccer game that starts at the center). So, the fifty-yard line separates the "home-visitors" zones, as you are on the field and at it, you're in the middle point from your end zone to the opponent's. Spectator seats closest to the fifty-yard line offer the best view of the game.
Whether "he" was a player or a spectator should be clearer from the context.