Railway interlocking is designed to prevent trains from colliding. Generally speaking current systems are safe due to the processes that signalling systems have to follow as well as implementation procedures that interlockings are compliant with. Even the processes and procedures are very advanced; there are still train crashes that are triggered by interlocking faults. However, the interlocking faults are mainly related to poor maintenance of the systems or/and changes to the original designs without correct authorisation. While those areas are worth exploring to search some improvements it is also important that interlocking will progress its development toward intelligent transport system reinforcing maintenance functions. Rail signalling systems went toward eliminating human factor by introducing machine based decision making on whether to proceed or stop. Development of electrical and electronic systems found interlocking moving toward relay solutions that became very common and are still used on railways. Finally computer based interlocking was introduced and successfully implemented in 90's. Interlocking logic of those solutions has not been formalised so that it can be progressed toward intelligent transport systems. Interlocking functions are limited to ensure train operations are safe. Maintenance functions (e.g. fault prediction) or functions to enhance passengers' management are currently not implemented in rail interlockings. This paper elaborates on currently implemented rail interlockings, compares various technologies implemented in those interlockings and proposes a base for consistent development of formal interlocking methods.