The 'SubwaySim 2 - Berlin Route DLC: U4' brings one of the capital's shortest underground lines to the simulator. Connecting Innsbrucker Platz to Nollendorfplatz, the route spans just 2.9 kilometres and can be driven in approximately six minutes. Despite its short length, it offers a highly detailed recreation of the real-world U-Bahn infrastructure.

Developed in collaboration with 3DZug, the add-on focuses heavily on environmental authenticity. Each of the five stations features meticulous detailing, including real-world materials, lighting, signage, and passenger information systems. The tunnel sections also introduce dynamic lighting that responds to the ongoing operational situation rather than remaining statically illuminated.

Operations on the U4 are fully integrated into the game's existing timetable and career modes, supporting both player-controlled and AI services. A completely new signalling system has been implemented specifically for this line to closely mirror actual operational procedures. Additionally, players can tackle four unique scenarios that present unexpected challenges.