The railway structures need constant monitoring and maintenance to ensure the train circulation safety. Quality information concerning the infrastructure geometry, namely the three-dimensional linear elements, are crucial for that processes. Along with this work, a method to automated extract three-dimensional linear elements from point clouds collected by terrestrial mobile LiDAR systems along railways is presented. The proposed method takes advantage of the stored cloud point's attributes as an alternative to complex geometric methods applied over the point's cloud coordinates. Based on the assumption that the linear elements to extract are roughly parallel to the rail tracks and therefore to the system trajectory, the stored scan angle value was used to restrict the number of cloud points that represents the linear elements. A simple algorithm is then applied to that restricted number of points to get the three-dimensional polylines geometry. The obtained values of completeness, correctness and quality, validate the use of the methodology for linear elements extraction from mobile LiDAR data gathered along railway environments.Infrastructures 2019, 4, 46 2 of 15 the need for pedestrian access to the railway, while gathering dense and precise information of the railway surrounding area. There are essentially two MLS installation solutions to collect the point clouds along the railway: mounted directly on the train or mounted on a car transported by the train. Despite the different advantages and disadvantages of those different solutions, there are no significant accuracy differences in the obtained cloud points [1].However, the MLS gathered point clouds are non-selective, i.e., they represent all of the elements in the sensor surrounding at a given moment, without any sort of classification. This makes it impossible to distinguish between points that represent the ground, vegetation, poles, rail tracks, persons or any other object within the sensor range. It is then the necessary method's implementation that allows an efficient classification of the points that represents a specific object among the millions of the cloud points.In last few years, the use of LiDAR data has progressively increased, and several studies have been published where this type of data are used for railway infrastructures monitoring, such as tunnels [2,3], railway gauge [1], railway bridges [4], rail surface monitoring and quality indexing [5] and rail surface defect detection [6]. Some studies have focused on extracting point elements from MLS data, like catenary systems [7], poles [8], trees [9] and traffic signals [10]. In [11], previous existing algorithms are adapted to extract several nonlinear objects from the railway environments, namely buildings [12], electric poles and vertical signals [13], trees [14] and ground points [15].Regarding linear elements extraction, the automatic extraction of railway tracks lines is clearly the most recurrent subject in the existing bibliography about the use of MLS point clouds in railway envir...