## 1 Introduction  
Railway networks are critical infrastructure to national economies. However, the integrity of train rails faces continual challenges due to mechanical stress and environmental factors, leading to various defects. Foremost among these is rolling contact fatigue (RCF), worsened by acceleration, heavy loads, and extensive railway usage. RCF defects appear as distinct 'fish-scale' patterns, visible on the rail surface or within its internal structure, leading to layered and overlapping defects. Over time, these defects propagate at inclined angles into the subsurface, posing significant risks to railway integrity. Track breaks caused by RCF require regular monitoring due to chronic environmental hazards inherent in railway operations [1, 2]. Hence, continuous monitoring of RCF is essential for ensuring public safety and the uninterrupted functioning of railway systems.  
Various non-destructive testing (NDT) methods have been developed to detect RCF defects. Ultrasonic testing (UT) [3] has proven effective in identifying internal rail defects. However, UT requires couplers and cannot detect surface or sub-surface defects [4]. Machine vision (MV) systems [5] excel at detecting surface defects but are less capable of identifying subsurface flaws. Electromagnetic detection methods [6, 7] provide a comprehensive solution for detecting both surface and sub-surface defects in railway infrastructure. Eddy Current Pulsed Thermography (ECPT), combining eddy current and infrared thermography, innovatively detects both surface and sub-surface defects in rails, offering visual indications through infrared thermography. ECPT is currently gaining significant interest and adoption for detecting RCF in railway infrastructure.  
The sensor structure of ECPT significantly influences the distribution of the electromagnetic field on the surface of the test specimen, thereby influencing the inspection results. Excitation coils play an important role in the ECPT system. In recent years, a lot of studies have focused on optimizing detection effects through adjustments to excitation coil parameters, positioning, and structural modifications within the ECPT detection framework. A variety of excitation coils have been applied in ECPT inspection systems. However, it is noteworthy that these coils are not specifically designed to simultaneously address both the 'field of view' and the 'heating effect' requirements.  
In addition to excitation coils, the excitation frequency plays an important role in the inspection result. Due to the skinfold effect, higher excitation frequencies are used for detecting surface cracks, whereas lower frequencies are utilized for detecting sub-surface cracks. As the depth increases, the eddy current skin depth becomes much smaller compared to the depth of the defect, resulting in the concentration of heat primarily on the surface of the specimen [8]. Consequently, single-frequency experiments may not yield optimal results. In order to obtain information at multiple depths, higher skin depth is required, enabling the acquisition of crack information across different layers of the specimen.  
Section 2 introduces the previous studies, analyzes their shortcomings, and presents the improvements and innovations of this paper. Section 3 provides a detailed elucidation of the methodology of sweeping ECPTT and the process of feature preparation. Section 4 describes the experimental platform setup and the specific design of the five types of coils. Section 5 presents the thermal images and includes a quantitative analysis of the results. Finally, Section 6 concludes with a summary of the key findings, their significance, and future work.