# Wheel Flats in the Dynamic Behavior of Ballasted and Slab Railway Tracks
## 1 Introduction  
Railway tracks can be categorized into ballasted and ballastless or slab systems. Ballasted tracks are composed of rails supported by sleepers that are placed over a ballast layer. The rails are connected to the sleepers by fastening systems. The pads, together with the ballast, the sub-ballast layers, and the foundation, provide elasticity to the system. Regarding slab track systems, the International Union of Railways [1] considers seven different families:  
1. Systems without punctual fastening of the rail;
2. Systems with punctual fastening of the rail and independent stretches of rail;
3. Systems with punctual fastening of the rail on sleepers incorporated into the structure by infill concrete;
4. Systems with punctual fastening of the rail on sleepers incorporated into the structure by vibration;
5. Systems with punctual fastening of the rail on sleepers laid and anchored on a supporting structure;
6. Systems with punctual fastening of the rail on sleepers separated from the supporting structure by a resilient level;
7. Systems with punctual fastening of the rail on prefabricated slabs. Slab track systems may be interesting for high-speed and heavy traffic lines. In Europe, one of the most used slab systems is Rheda 2000, a continuous track, which belongs to the third family. Due to this reason, in this research, a continuous slab track system was studied. Wheel flats are the most typical type of wheel defects during long-term train operation. These defects induce high-impact loads on the railway infrastructure, which causes significant damage to both vehicles and tracks, such as broken axles, hot axle boxes, damaged rolling bearings, and cracks on the wheels, the rails, and the sleepers. Furthermore, high levels of noise and excessive vibration are generated by this type of wheel defect. Wheel flats are mainly due to anomalous wear of the wheel due to sudden braking or due to insufficient adhesion force between the wheel and the rail [2]. Although this subject has been studied both numerically and experimentally over the last few years by several researchers, the wheel flat problem remains of current interest because the vehicle speed is increasing, contributing for higher dynamic loads (due to speed and irregularities) on the tracks. Furthermore, there are limited studies on the effect of wheel flats on slab tracks. Wheel flats induce high-impact loads, which should be controlled and limited to maximize the life of the system. In recent years, many different methods for measuring in situ wheel defects have been proposed. For instance, [PERSON] et al. [3] tested two vibration signal processing methods for the detection and diagnosis of wheel flats: the cepstrum analysis method combined with order analysis and the cross-correlation analysis method combined with order analysis. [PERSON] et al. [4] concluded that the envelope spectrum analysis is a capable tool and a cost-effective method that can be used to detect wheel flats along with the flat impact frequency for different train speeds in real-world conditions. [PERSON] et al. [5] ran a 3D numerical simulation of the train track dynamic response to the presence of wheel flats to identify the type of sensors that can be adopted in wayside wheel flat detection, but this study focused only on ballasted tracks. The EU-funded MAXBE Project ([[http://www.maxbeproject.eu/](http://www.maxbeproject.eu/)]([http://www.maxbeproject.eu/](http://www.maxbeproject.eu/)), accessed on July 2021), which focused on detecting wheel flats and axle bearing failure modes at an early stage [6], also presented relevant methodologies for wayside and on-board vehicle monitoring. To detect and diagnose wheel defects, the problem has to be fully understood both experimentally and numerically. The most relevant and recent contributions to the wheel flat problem [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19] focused on ballasted tracks, and studies on slab track systems are lacking.