VEPS has been under development since 2002 in Estonia. Based on programmable logic controller technologies, the system was developed by Estonian Railway engineers and patented in 2004 by Indrek Syld. The new system was installed in the CE35 type of locomotive (77 systems) during 2003-2004. A new platform (level 2.0) was installed on Stadler's FLIRT trains for Estonia in 2012 (76 systems), and in 2016 a third generation of the system was introduced for the international market. KLUB Since the 1990s, the Russian Railroad Company has introduced a computerized successor system KLUB-U which requires either ALSN only or both, ALSN and ALS-EN sensors for compatibility. In ERTMS the ALSN/ALS-EN systems are listed as ETCS Class-B systems. ALS-ARS A similar in general theory of operation, but differently implemented, is an ALS-ARS system used in subways of the former Soviet Union. It uses low frequency signals transmitted and received as described above, with the difference that the signals are continuous wave and the signal's frequency determines its code meaning, which results in much faster response time. Typically, two frequencies are transmitted at the same time, indicating the current speed limit and, if lower, the limit for the next section, providing the train driver with a warning and time to react and reduce speed. In case of an overspeed, the train is unconditionally braked to the indicated speed limit, and then, if not acknowledged by a driver pressing a vigilance button, to a complete stop. Should be there no frequency received (such as on an uncoded track in a maintenance depot or in case of an equipment failure), the driver may proceed at a low speed while holding a vigilance pedal. Train protection systems Railway signalling in Russia