Simplify the following text for a Grade 9 native English-speaking person.
Original: The service was an immediate success , reaching the 100 million passenger mark in less than three years on 13 July 1967 , and one billion passengers in 1976 .
Simplified: The line was an immediate success , reaching the 100 million passenger mark in less than three years on July 13 , 1967 and one billion passengers in 1976 .