## 1 Introduction of Transition Zones  
Transition zones in railway track network are locations with considerable changes in track support structures. Typically, they are located near concrete structures, such as bridges, culverts, and tunnels. An example of a bridge transition zone is shown in Figure 1.  
Figure 1: A typical bridge transition zone. The track geometry in transition zones degrades at a much faster rate than that of open tracks [1; 2; 3; 4], which results in the appearance of a \"dip\" [5]. This phenomenon has been confirmed by a survey of the performance of track transition zones, which revealed that 51% of the track transition zones had experienced such a settlement [6]. In the field measurement of a bridge transition zone [4], the rail displacements on multiple locations were measured during train passages. Results showed a 5.1 mm deep dip in the ballast track. A dip was also found in the transition zone of level crossing in [7], which was 5.3 mm in depth. Such large geometry irregularity leads to an increase of dynamic wheel loads [8] and the redistribution of dynamic wheel loads [3]. The increased dynamic wheel loads may damage track components, for example [1; 2; 3; 6; 9; 10; 11; 12; 13; 14; 15; 16; 17]:  
* rail surface defects and cracks in the rail foot;
* broken fasteners;
* cracks in concrete sleepers;
* breakage of ballast particles and voids between sleepers and ballast, also known as hanging sleepers. It also initiates the further deterioration of track geometry, causing a self-perpetuating system [15; 17; 18; 19; 20; 21]. Increased dynamic wheel loads can result in worsening of the passenger's comfort [18], and can even create a potential for train derailment. As a result, transition zones require more maintenance, like tamping and adding ballast, as compared to open tracks [11; 22]. For instance, in the Netherlands, maintenance activities on the track in transition zones are performed up to 4-8 times more often than on open tracks [14; 23]. In the United States (US), $200 million is spent annually on maintenance of the track in transition zones, while in Europe, about 697 million is spent on similar maintenance activities [24; 25]. The major factors causing transition zone problems can be divided into three categories [1; 6; 22]:  
* Significant abrupt changes in the vertical stiffness of the track;
* Differential settlement or uneven profile of the ballast track, which inherently settles more than a concrete structure;
* Geotechnical issues, such as poor drainage conditions and poor quality of materials. Many studies have been conducted to analyse the former two factors in transition zones, such as in [6; 8; 11; 17]. A thorough review can be found in [26]. The geotechnical factor has been studied less, which is mainly due to the limitations of measurement methods. For instance, the inspection of the ballast quality was typically conducted by excavating samples from ballast layers, which is costly, destructive, and needs track possession time. The moisture condition of the ballast may play an important role in the rapid degradation of tracks and consequently geometry irregularities. Excess water in substructures significantly reduces (around 50%) the resilience of tracks [27; 28] and increases (around 40%) track settlement [29; 30; 31]. The loss in resilient modulus and the increase of settlement have significant implications for transient vertical displacements and track performance. This paper presents an experimental analysis of the moisture condition to explore the relationship between moisture and track condition in transition zones. In total, three transition zones of various conditions are analysed. The moisture condition in the transition zones is measured by the use of ground penetrating radar (GPR) [32; 33], which is compared to the track geometry that is measured by inspection coaches. The relationship between moisture condition and geometry irregularity is analysed. Transition zones in high moisture condition are numerically studied using the Finite Element (FE) method.