Simplify the following text for a Grade 9 native English-speaking person.
Original: They were initially classified as AM3 units before the introduction of the TOPS classification system , and were the dominant EMU on the Glasgow suburban railway network for over 25 years before being progressively phased out by newer rolling stock .
Simplified: At first they were classified as AM3 units before the introduction of the TOPS classification system . They were the dominant EMU on the Glasgow suburban railway network for over 25 years .