# Influence of Stray Current on Fastening System Components in Urban Railway Tracks
## 1 Introduction  
Owing to their high capacity and velocity, urban railway tracks are the main type of transportation in many cities worldwide. Currently, urban track systems mostly use the DC traction power supply mode, and rails are normally used as return conductors from vehicles to substation [1, 2, 3, 4]. The electrical resistance of the rail results from creating a potential drop in the rail, and because it is impossible to completely insulate the rail from its surroundings, part of the current leaks into the track bed and soil [1, 5, 6, 7]. This current is called stray current. Stray current enters nearby metal objects that have a lower resistance than soil, such as buried metal pipelines or steel in reinforced concrete, and flows back to the source [8, 9, 10]. The current path through a rail or other metal object can be very long and not harmful; however, stray current corrosion starts at the point where the current leaves the metal and enters the electrolyte [11, 12]. Stray current corrosion became a source of concern for transit authorities and utility companies immediately after the first electrified tracks began to operate. Initially, it was thought that the corrosion problem was caused by a chemical mixture of the soil; however, after some research, it became clear that soil alone could not cause the severe corrosion found in rail bases and nearby utilities. It was concluded that the cause of this corrosion was a stray current [13, 14]. Stray current corrosion is an electrochemical process that involves two simultaneous reactions. A cathodic reaction occurs in the cathodic zone, where the current enters the metal structure or rail, whereas ananodic reaction occurs where the current leaves the rail or metal structure and enters the soil or other electrolyte, called the anodic zone [15; 16; 17; 18; 19]. Corrosion and metal loss occur in the anodic zone. Currently, most studies [20; 21; 22; 23; 24; 25] focus on stray current corrosion in pipelines or on stray current simulations and modeling [7; 16; 26; 27]. The main objectives of some conducted studies are listed in Table 1. Even though minor corrosion damage is sufficient to compromise the integrity of pipelines, it is not surprising that many researchers have focused on analyzing the detrimental impact of stray current corrosion on pipelines; however, stray current can also have detrimental effects on urban tracks and lead to the degradation of rails and rail fastening systems. Corrosion itself does not cause a significant mass loss in the track and is generally hidden and difficult to detect by visual inspection [28]. This could be one of the main reasons why rail corrosion has not yet attracted sufficient attention and has become a major concern for operators. However, a small loss of rail material can lead to a significant change in the material properties or even act as a \"stress concentrator\" for the development of cracks [28; 29; 30; 31; 32]. Conditions such as sunlight, relative humidity, temperature, and atmosphere have major impacts on corrosion. In addition, salt deposition is an important factor that increases the corrosion rate [33]. According to [PERSON]'s law of electrolysis, a current of 1 A results in a loss of 9.1 kg of steel per year [26]. The EN 50122-2:2022 standard [41] states that experience indicates that no damage occurs in tracks if the average stray current per length does not exceed 2.5 mA/m. The methods used to reduce the stray current at the source can be divided into two groups [19; 20; 42; 43]. Decreasing the longitudinal electrical resistance of the rails, and  
Increasing the rail-to-ground resistance. The shape and material properties of rails are mainly selected according to mechanical requirements, whereas the electrical properties are related to the background [44; 45; 46]. However, the longitudinal electrical resistance of the rail can be reduced by methods such as using rails with a larger cross section and reducing the distance between substations [47].