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Answer Wiki. Related QuestionsMore Answers Below. The white cylinder in the foreground is an hydraulic accumulator to allow the level crossing barriers to operated once or twice in the event of a main power or hydraulic pump failure. The brick huts include battery backup for all the signalling and barrier systems between Peterborough and Spalding. The final level of redundancy, if systems fail completely, would be human. A group of Flagmen would control the traffic, and a token system would replace signalling. The token could even be in the person of one flagman, accompanying the driver. The disruption to timetables would be severe, and this scheme would only last until a rail-replacement-bus service could be cobbled together, allowing complete closure of that portion of the route on safety grounds. It should be noted at the outset that whatever the requirements for standby power are as set by state governments or even by the AAR, the railroads in fact set more stringent requirements upon themselves. In the back of the mind of every signal engineer that was contacted in the course of this investigation, lurked the danger of primary power to a grade crossing failing, the batteries running down to vagaries of the maintenance schedule, and an accident resulting at a then unprotected crossing. Such fears are fueled by the fact that isolated situations like this have occurred. Therefore, very conservative design criteria are used in determining required storage battery capacity for providing standby power at a given crossing. In some states, every grade crossing warning system installed must pass the scrutiny of a state safety engineer who essentially imposes a formally unwritten requirement for some period of normal operation. The 72-hour rule originally adopted by New York is one such popular rule, in spite of the fact that New York has shortened the requirement. Once again, it should be noted that railroads themselves always exceed, and sometimes greatly exceed, these requirements. Energy And Power In Signals. A signal which satisfies the condition 0 See more Electrical Engineering topics.