## 2 State of the Art  
Since the 1960s, ultrasound technology has been widely employed as a non-destructive testing (NDT) technology to identify railway flaws [11]. Typically, sub-surface cracks are identified by monitoring the signal strength change in reflected ultrasonic waves via a pulser-receiver system [3]. The instruments employing such technology range from manual hand-held devices to automated rail cars [12]. The current industry standard utilises fluid-filled ultrasonic wheel probe technology with piezoelectric transducers [13]. The phenomenon of propagation of ultrasonic energy in solid materials, in the form of transverse (shear) waves and longitudinal (compression) waves, compounded with the correlation of the wave's confinement/transmission in the material with wavelength and its exponential decay with depth, are the basis of all the studies related to NDT technology [14]. In 1953, the first Sperry test vehicles were introduced, supplied with ultrasonic transducers for high-speed rail inspection systems [12]. Research dating back to the 1960s shows extensive studies performed on wave-associated displacement profiles and wave scattering in relation to variance in material integrity. Over the years, technology development led to the two typical configurations of ultrasonic transducer inspection methods: the pitch-catch and the pulse-echo. The pitch-catch configuration comprises a separate actuator and sensor, whereas when using the pulse-echo method, the transducer acts as both the actuator and sensor. Typically, pulse-echo transducers are applied for the detection of sub-surface flaws, while pitch-catch transducers are used for the detection of cracks/discontinuities such as vertical splits (parallel to the rail length) [15]. For both transducer configurations, to aid wave transfer between the sensor and rail track, a liquid-based couplant is required. In 1967, [PERSON] proposed measuring the change in period of oscillation in the frequency domain to determine the defect depth [16]. This approach was enhanced further by [PERSON] et al. and [PERSON] et al., whose research facilitated the determination of defect length via pulse time measurement [17; 18]. In the research conducted by [PERSON] et al., it was concluded that defect detection is optimised when transmitting ultrasound waves at incident angles of 0\({}^{\circ}\), 37\({}^{\circ}\), 45\({}^{\circ}\) and 70\({}^{\circ}\)[19]. This approach makes ultrasonic flaw detection highly efficient, especially in recognising deep internal defects, particularly those located in the rail head, web and foot regions. However, the configuration is less efficient for identifying rolling contact fatigue defects typically located closer to the surface (<4 mm deep) [19]. Researchers have investigated the effect of frequency during laser-generated ultrasound. Frequency analysis studies have indicated that frequencies between 0.4 and 3 MHz used in combination with a high-order wave are suitable for the measurement of the cold work layer and stress measurements [12; 20; 21]. [PERSON] et al. and [PERSON] also analysed the resulting ageing process in steel and defect flaws through examining changes in wave velocity and attenuation of high-frequency waves above 1 MHz [22; 23]. In a study by [PERSON] et al., the limitation of high-frequency analysis for the application of deep detection of flaws was identified and attributed to a lack of sensitivity at deeper defect depths [24]. In a further study by [PERSON], it was found that frequencies of 2 MHz are effective for defect detection to a depth of 0.75 mm. However, at lower frequencies of 150 kHz, defects can be identified at depths of 10 mm [22]. Low-frequency waves, however, can experience a loss of resolution [25]. In particular, in a study by [PERSON] et al., it was observed that when analysing defects that measure 15% of the rail head section, reflections are identified when using a 20-45 kHz frequency range [25].