You'll have to be creative here, since it's not a conductor. It's a person responsible for the safety of passengers, order, etc. in the car (like a hall monitor in schools :-)

Something like this: Private First Class (or whatever you call him) may be assigned a supervisory function/role in the car...

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Note added at 3 hrs (2013-09-30 01:08:27 GMT)
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Спасибо, Давид. Я очень люблю эту тематику. Она мне тоже доставляет удовольствие.