Because tunnels block a radar signal, trains rely largely on hard-wired equipment like the box beside the track, which helps drivers and dispatchers talk to each other and controls a machinelike traffic signal, giving the drivers basic commands to stop and go.

NEWYORKER: Boss Rail

Engineers used plastic dams in a bid to stop water getting into signal boxes at Cowley Bridge, Devon.

BBC: South West rail problems could last five days

That would stop the radio signal needed to take money off your bankcard.

BBC: London bus contactless system charging wrong cards

Obama also made clear that Israel has the right to defend itself as it sees fit, which amounted to a diplomatic signal that Washington would not stop a unilateral Israeli strike at some future point if no progress occurred.

CNN: Obama, Netanyahu agree on preventing nuclear-armed Iran

On the night of July 23, 2011, when trains began to stack up, dispatchers and maintenance staff raced to repair the faulty signal and ignored the simplest solution: stop the trains and regain the signal.

NEWYORKER: Boss Rail

When you stop the shopping addiction, the stories within you signal to themselves that the way is clear, that the suffering is over.

FORBES: Reverend Billy and His Crusade Against Consumerism

When the signal at Gadadharpur Halt turned green, the Uttar Banga Express had moved on to its next scheduled stop of Sainthia.

BBC: Driver 'error' led to West Bengal train crash

This system, Positive Train Control (PTC), is intended to stop or slow a train automatically if a driver goes too fast or passes a red signal.

ECONOMIST: American railways