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Railway freight wagons are a prime target for thieves, so some have tracer
beacons that allow them to be tracked. But infrequent servicing means these
batteries often go flat. Now the Sema Group in Britain has a solution. A
transmitter on the locomotive sends out a “token”, a weak radio signal that’s
bounced down the train’s wagons. When wagon 1 receives the token, it retransmits
it as token 2, which wagon 2 uses to generate token 3 and so on, down to the
last wagon, which then sends the tokens back up the train. If any wagon is lost
or stolen, the tokens don’t tally (GB 2 336 011).