The maximum transmissible electrical power concept is applied to different electrical railway systems. Different problems arise depending on whether the electrical system is using DC or AC system. A DC example illustrates how the power line loadability can be assessed. Afterwards, the definition is extended to AC system to take into account the effect of the reactive power. Finally, an AC use case with an high voltage booster shows the importance of these phenomena and some difficulties to properly simulate a system close to its limit.