Simplify the following text for a Grade 9 native English-speaking person.
Original: It was the first underground railway ( or subway ) in the world , opening on 10 January 1863 ( however , parts of that initial section are no longer served by the Metropolitan line , but by the Hammersmith & City , District and Circle lines ) .
Simplified: It was the first underground railway ( or subway ) in the world , opening as the Metropolitan Railway on 10 January 1863 . ( Parts of that initial railway are no longer served by the Metropolitan line , but by the Hammersmith & City , District and Circle lines . )