1. Introduction

' The past decade has witnessed the most prosperous blooming of HSR, marking a splendid new age of this fast-developing transportation, which subtly alters people s travelling habit with great convenience and ride comfort. Hidden behind the highquality ride service provided by HSR is the tremendous effort and huge budget spent on the inspection and maintenance work, which is more challenging with increasing speed and capacity.

' With long-term numerous cycles of loading and unloading, both rail tracks and train wheels are suffering from vibrations and stresses caused by wheel/rail interactions, leading to fatigue, wear, plastic deformation, cracks and other deteriorations. The wheel/rail interactions are intense with average contact stresses over 1000 MPa under normal operating conditions, and this number can go much higher upon specific situations (wheel flange/rail edge contact while train turning, poor conforming wheel and rail profiles, etc.) [1]. Moreover, to authors knowledge with recent research work on contact mechanics using NDT approaches, machine element contacts including wheel/rail contacts are essentially contacts between the

asperities due to surface roughness of the contact bodies, and the asperity contacts indicate hyper-stress concentration beyond 4000 MPa at the contacting peaks [2]. Under such high stresses, components of the rail system are deteriorating rapidly in various forms and the deteriorated structures create a worse operating environment, adding the occurrences of failures. A typical failure is rolling contact fatigue (RCF) causing a series of subsequent rail defects (squats, transverse cracks, spalling and gauge corner cracks).

The rail also takes up impact load from running trains intermittently due to wheel defects, rail irregularities or at certain areas rail turnouts, rail joints, etc. The intense vibrations caused by wheel/rail interactions and impacts are transmitted bidirectionally from the wheel/rail interface up to the coach and down to the rail slab simultaneously. In terms of HSR, to meet the high standard requirements of smooth operation under high speed, the components utilised are different from those in conventional rail lines. For example, the rail tracks are strengthened with high resistance to wear, and multi-layer concrete forms up the rail slab with CA mortar layer serving as the damping instead of traditional ballast. These measures add ride comfort in HSR operation, but make the system more 'brittle' with reduced capability in vibration absorption, hence add the risks of cracks in the rail system. A recent example is the giant crack (44 cm long) found in an operating Japan Shinkansen bullet train in December 2017, causing interruption of service and great social panic [3]. Similar cases can be highly possible on ballastless rail tracks leading to more catastrophic consequences, calling for more reliable and thorough inspection actions.

NDT techniques have long been used for inspection in rail system since the 1920s. With integrated ultrasonic probes or eddy current sensors, the NDT systems are able to check surface and internal defects along the rail in either contact or noncontact manner. The NDT inspection is conducted through manual inspection device or inspection vehicle. Conventional inspection vehicles are normally attached to a traction locomotive to carry out inspection. In the age of HSR, many countries have developed high-speed comprehensive inspection vehicles (CIVs) for the more complicated inspection tasks, such as the 'East-i' CIV in Japan, the 'IRIS320' CIV in France, and the 'No. 0' CIV in China, etc. Inspection content of the high-speed CIVs covers from geometry data of rail infrastructure to dynamic behaviours of trains. Despite of the wide range of data types, the NDT techniques require interruption of train service to conduct the inspection. To provide early alarming in prevention of further consequences in terms of accidents similar to the Japan Shinkansen case, continuous real-time information of in-service rail system is highly desired, which puts forward the introduction of online monitoring to this area. Since wheel/rail interaction is the core part of the rail system, this chapter mainly focuses on the inspection and monitoring methods of wheel and rail defects.
