Continuing from the BD line the first stage would lead from Dietikon to Spreitenbach and Killwangen.
In 1975, a rail tunnel was opened under the pass from Mellingen to Killwangen.
It is electrified at 15 kV 16.7 Hz and its eastern 16 km section from Zürich to Killwangen-Spreitenbach is now part of the Zürich–Olten trunkline and has four tracks.