Abrupt track vertical stiffness variations along railway tracks can lead to increased dynamic loads, asymmetric deformations, damaged track components, and consequently, increased maintenance costs. The junction of slab track and ballasted track is one of the existing areas where vertical track stiffness can suddenly change, therefore requiring a transition zone that smoothes the track stiffness change. One of the methods for constructing the transition zone at the junction of slab and ballasted tracks is to install auxiliary rails along the transition zone. In the present study, the dynamic behavior of this type of transition zone was evaluated by a train-track interaction model. For this purpose, a 3D model of the railway track was made, representing the slab track, the transition zone, and the ballasted track. Then, the modeling results were validated by the results of field tests. Afterwards, in order to study the dynamic behavior of the transition zone with auxiliary rails, different sensitive analyses, such as vehicle speed, vehicle load, number of auxiliary rails and railpad stiffness, were performed with the model. The obtained results showed that the use of auxiliary rails reduced the rail deflection variations along the transition zone from 35% to 28% for low and medium speeds (120, 160, 200 km/h), and from 40% to 33% for high speeds (250, 300 km/h). KeywordsRailway track, transition zone of slab track to ballasted track, auxiliary rails, 3D train-track interaction model.

Train-induced vibrations have adverse effects on the structure of a track and its surroundings. It is thus necessary to study and control this issue through efficient measures. Special polyurethane pads that are placed under sleepers have been recently suggested as a solution to this problem. The use of these pads reduces vibrations at their source. These pads, called under sleeper pads (USPs), have numerous advantages and have found wide application in the railway industry. However, very few studies have been conducted on USPs; thus, this paper presents a field study that documents their effects on reducing vibration levels. Fifty stiff and 100 soft pads were prepared with the required technical and mechanical specifications. They were then installed onto concrete sleepers during the production process. These sleepers were installed at km-253 of the Mashhad–Tehran track. Train pass-by vibrations of passenger and freight trains were recorded using geophones placed at 0, 3, 7, 11, 15 and 19 m distances from each track axis (one track with and one without USPs). The results of the tests show that the maximum root-mean-square velocity was reduced by about 60%. Frequency analysis of the results demonstrates that the vibration reduction effect of the USPs appears above 40 Hz. The best efficiency levels of the USPs were observed in the range 40–80 Hz and the maximum USP efficiency occurred at the frequency of 50 Hz. At this specific frequency, the maximum vibration velocity level was decreased by more than 6.7 dB for all the considered distances (0, 3, 7, 11, 15 and 19 m).

An abrupt change in the stiffness of railway tracks at the junction between slab track and ballasted track causes increased dynamic loads, asymmetric settlements, damage of track components, and, consequently, increased maintenance costs. Due to this, a transition zone is usually built at the junction between the ballasted and the ballastless tracks to reduce the aforementioned problems. One of the methods suggested as a transition zone in these areas is to use a combination of an approach slab and additional rails. This study evaluates the dynamic behavior of this type of transition zone using field tests and also compares its performance with a transition zone built only with an approach slab. Hence, in the Tehran–Karaj railway line, two types of transition zones were constructed: one including only the approach slab and the other one including additional rails and an approach slab. Then, by conducting some field tests, the dynamic behavior of the track in these two types of transition zones was examined. The results of the field measurements show that for the analyzed case study, at the combined transition zone with additional rails and an approach slab, instead of a sudden increase in rail displacements from the slab track to the ballasted track (i.e. by 53%), initially, in the first part of the transition zone (with additional rails and an approach slab), the deflections increase by an average of 31%, and then in the second part of the transition zone (with additional rails only) the deflections increase additionally by 11%.

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Culverts and bridges are the most common areas on railway lines where abrupt changes of track stiffness can occur. Using auxiliary rails in the transition zone is one option for gradually increasing track stiffness. The study reported in this paper investigated this issue by means of finite-element modelling. A ballasted track with a concrete culvert on the Tehran–Isfahan high-speed railway line in Iran was modelled and the effect of installing auxiliary rails in the transition zone was investigated. The ballasted track and its components – including railpads, ballast and subgrade – were modelled as a series of mass–spring–damper systems, while the sleepers and culvert were simulated using Euler–Bernoulli beam elements. The dynamic behaviour of the track and its components was investigated by simulating loads moving at different speeds. The results indicated there was a reduced variation in rail deflections, track accelerations and railpad forces, demonstrating that auxiliary rails improve the dynamic performance and behaviour of transition zones.