with the fact that all S-train stock had similar braking characteristics, allowed the assumption of reduced braking distances and the use of fixed blocks significantly shorter than on conventionally-signalled railways. Lineside signals were provided for use when the HKT system was inoperative or when a non-equipped train needed to be driven on the line, but line capacity was significantly reduced as these signals only protected full-length blocks and had to be placed according to conservative braking distance assumptions. In 2011, infrastructure operator Banedanmark awarded Siemens Mobility a contract to replace HKT with its Trainguard MT communications-based train control system (CBTC), on the grounds that the HTK installation was reaching the end of its life and becoming increasingly unreliable. The CBTC system employs the moving block principle, whereby a central controller continuously tracks the precise location and speed of every train on the line and adjusts the instructions issued to drivers accordingly. On the S-train network this has allowed a reduction in minimum separation between consecutive trains ('headway') from 120 seconds to only 90. Hillerød–Jægersborg was the first section to be converted, as its F-HKT installation ('simplified' HKT) had the oldest hardware and did not deliver the same level of train protection as the other parts of the system. The new CBTC system entered service on this section on 29 February 2016. Further conversions continued through to 2022, when in late September the final three routes (Valby–Frederikssund, Valby–Høje Taastrup, and Sydhavn–Køge) were switched over to CBTC operation. Rolling stock The S-train has seen four types of rolling stock. Currently, only fourth-generation trains operate, after the retirement of the second and third generation trains in 2007 and 2006 respectively. First generation (1934-1978) The first generation (DSB class MM-FM-MM) was introduced in 1934 at the opening of the network, and consisted of three carriages per train set (2 motor cars and 1 trailer car), manufactured from 1934 to 1962. Frichs A/S supplied the electric components while Scandia (now Bombardier Transportation Denmark) supplied the car bodies and non-electric components. They were retired in 1978, and one was used as a heritage train until 2003, when DSB decided to discontinue its use. They are the longest serving S-train rolling stock to date, in continuous service for 44 years. Second generation (1967-2007) The second generation (DSB class MM-FU-MU-FS) was built from 1967 to 1978 by Frichs A/S (motored cars) and Scandia (trailer cars). They saw the