The long history of Europe's railways, starting from the first industrially used railroads in 19th century England, evolved in a very diverse manner. Until now, it has been shaped by a great number of national characteristics which go far beyond the differences in rail gages. For example, there are:
Different regulations concerning the operating procedures and separation of responsibility for operating and maintenance staff (interlocking systems staff, train drivers).
Incompatible power supply systems (AC/DC, combined with different voltages and AC frequencies).
Diverse braking systems and energy recovery systems in high-speed trains.
Different rail profiles and installation positions (angles of inclination) as well as differing maximum permitted axle loads.
Different catenary suspensions and incompatible structure gages (minimum distance between a virtual train corridor and parts which bound it, such as signal masts, buildings and oncoming trains).
This list is far from comprehensive. For example, there are currently 13 mutually incompatible Automatic Train Protection (ATP) systems in use within Europe.
These systems monitor the correct response of the driver to signal indications conveyed alongside the line or direct to the cab. If necessary, they intervene in the traction vehicle's drive control and brake control systems. It is these ATP systems, which were designed to work with country-specific signaling systems, that prevent cross-border train traffic in Europe. To overcome this problem, the European Union (EU) is defining railway signaling standards. This effort has been going on for more than ten years.