Moving block signalling

Moving block signalling could not effectively be implemented until the invention of reliable systems to communicate both ways between a train and a signalling system. While such technology has existed for decades, the impracticality of early systems made it unviable for many years. Pulse codes were used on the first version of the London UndergroundVictoria line's signalling system. However, a pulse code two-way communication system, using the computational technology at the time, would have been complicated, so a fixed block system was used instead.