Insertors were marked separately from the March 1903. They bore the letter E behind the line number of their main line. In later years, these lines increasingly took over the tasks of booster drives and were therefore shown in the timetables as separate lines. On April 15, 1912, the GBS introduced the first line with three-digit number. The 164 was created by extending the 64, which was maintained in parallel. In the following months more lines were provided with 100 numbers or newly set up, usually as a line pair to the existing line.