 Complete the following phrase:  With the old walls out of the way, the city developed quickly almost doubling in population over the following decade. The interconnecting railway line on the southern and western sides was replaced by the circle line railway in 1871, and the existing railway track was then used by horsecar lines. The route of the southern lines was used for the first electric metro line in 1900, which is now the U1 Berlin U-Bahn. Its metro stations Silesian Gate Schlesisches Tor, Cottbus Gate Kottbusser Tor, Halle Gate Hallesches Tor, and former Stralau Gate Stralauer Tor are a reminder of its heritage. The following streets follow the original route of the customs wall: Stresemannstraße former 