## 1 Introduction  
Railway safety is an ever-increasing issue, as traffic demand increases world wide with railways playing an important role. This is especially true in the light of the urgent need for the decarbonization of traffic. The accurate tracking of a train's real position on the track is the basis for all modern railway safety concepts. In state-of-the-art rail operations, the train's position is only known within a so-called \"block\" between two signals. In one block, there is only one train allowed to operate at a time, which is sufficient to prevent collisions of trains. The disadvantage of this concept is a reduced efficiency in the use of the available track infrastructure because long portions of the track stay unused. Future traffic demand calls for more flexible, thus efficient, concepts. Novel safety concepts will, therefore, be based on virtual- or moving-blocks that virtually enclose the train while in motion, providing enough safety separation between trains at all times [1, 2]. An accurate, reliable, and redundant train tracking technology is an important basis for these novel concepts. Conventional train tracking is mainly achieved by communication between train-side equipment, track-side equipment, and the interlocking system. Block-based systems need axle-counting sensors installed next to the track. They deliver a signal for each axle that passes through the detector, to ensure that a block is unoccupied before the next train enters this block. The European train safety system, ETCS, can use moving blocks in its level 3 implementation and relies on continuous positioning information of the train [3]. High accuracy differential satellite positioning systems (D-GNSS), such as GPS, Glonass, or Galileo, become available and are certainly planned to be applied in future railway systems [4], but the high safety requirements of railway operation call for highly reliable and redundant positioning systems. Furthermore, the coverage of D-GNSS and the necessary communication link back to the interlocking system is never 100% guaranteed: Tunnels, deep valleys, and tall metallic structures next to the track can degrade, or even block, radio frequency signals. Non-coherent optical time domain reflectometry (OTDR) technology has been used for a long time for long-range monitoring of the quality and the integrity of fiber optic cable infrastructure [5], e.g., to locate break points in underwater fiber cables. Distributed acoustic sensing (DAS), a special variant of coherent (or phase sensitive) OTDR, is not only able to assess the cable quality but also to detect and locate strain and temperature changes along the cable with high sensitivity. The measurement and signal processing methods presented in this paper are based on DAS and offer an alternative or redundancy system for accurate train positioning and tracking. Other than the aforementioned technologies, it does not need equipment installed on the train nor communication between train and interlocking system. Furthermore, it does not need extra track-side equipment, with the exception of a fiber optic cable that is already in place for data connection and communication purposes. In this paper, we investigate advanced signal processing of the DAS data received from the so-called optical interrogator device for accurate train tracking. The interrogator injects a series of laser light pulses into the fiber cable and measures the back scattered (Rayleigh backscatter) light at the same end of the cable. The measured signal contains the optical path length change resulting from a refractive index change due to compression of the glass and from contraction or elongation of the fiber due to ground deformation [6]. These effects can be received from any point along the cable over a range of up to 40 km and can be measured with a positional resolution down to 0.5 m. The reader interested in the different types of DAS devices and their physical principles is referred to the survey of [PERSON] and [PERSON] [7].