Infrastructure managers (IMs) endeavour to eliminate rail defects at an early stage since they impact on safety and quality of operation and increase system costs. London Underground (LUL) uses several non-destructive testing (NDT) techniques in rail inspection to detect the emerging defects and monitor the growth of previously recorded defects. This task mainly aims to prioritise maintenance and renewal activities and record their completion. However, when the high traffic demand and limited maintenance periods are considered, these requirements bring additional pressures to the maintenance team. To optimise maintenance planning, sufficient and reliable field data along with accurate damage prediction are required. Recent developments in NDT technology has seen the introduction of devices to measure crack depth which is a key parameter in the assessment of crack severity and rail life. Therefore, contrary to previous research which mainly utilised observations of rail surface condition, the use of new NDT techniques can support the development and validation of new rail damage models which will help to improve maintenance planning and move to condition-based maintenance strategy.