Simplify the following text for a Grade 8 native English-speaking person.
Original: Once the railway line was extended under the street to join the two , Princes Bridge slowly became amalgamated into Flinders Street .
Simplified: Once the railway line was extended under the street in 1865 to join the two , Princes Bridge was closed . It was not reopened until April 1879 , and from 1909 slowly became amalgamated into Flinders Street .