push-pull trains with diesel locomotives. However, the entrances to the trains were not barrier-free because of steps. Following the electrification of the line, it was replaced by a branch line of S2, and Class 420s were replaced as well in June 2017. Class 423 From the autumn of 2000, the class 420 trainsets, some of them over thirty years old, were replaced massively by new vehicles of the class 423. The new vehicle emissions were enforced line by line, as the trains could not be coupled with each other. First, the S 7 line was served by the new locomotives. By 2003, 211 locomotives were delivered. In 2004, another 23 units were procured. In 2005, four more trains were delivered, so today there are 238 traction vehicles. Others In 1972, during the Olympic Games, a consist made up of 15 Silberlinge cars and a DB Class 140 locomotive at the middle was used to haul trains on the S25 between the Ostbahnhof and the Olympiastadion stations. In 1989, the double decker trains used in the Netherlands were briefly used on the then S4 between Geltendorf and Ebersberg. Patrons were invited to fill out a questionnaire while on board describing their experiences while boarding and riding the train. The concept was to increase passenger capacity at the same time as accommodating bicycles, prams, and wheelchairs with efficiency. The trains were pulled by a DB Class 120 locomotive at each end of the train. Plans and further expansions The Second S-Bahn Tunnel (Zweite Stammstrecke) Nearly all lines use the core route through the city centre in the underground, creating a bottle-neck responsible for long and increasingly frequent delays from even the smallest disruptions. The disadvantage of current core route is the inability of trains to reroute themselves onto different tracks in an event of disruption. After years of discussions and studies into different route propositions, a second tunnel through the city centre has been already approved with the funding of €3.85 billion and the completion date of 2028–2032. On 5 April 2017, the ground-breaking ceremony took place to commence the construction. The second tunnel will be in length and run in parallel with the current tunnel in the south on the western portion of Hauptbahnhof and in the north on the eastern portion. It will have three underground stations (Hauptbahnhof, Marienhof, Ostbahnhof) and two surface stations (Laim and Leuchtenbergring). Marienhof, which will