Obviously for the question , you have to assume that the train's engine/head is at the pole. And after 12 seconds , the tail of the train crosses it .

So the length of train = distance covered by train in 12s

= 12/3600 * 120 = 1/300 * 120 = 12/30 = 4/10 = 0.4 km = 400m
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Please see the attachment.the answer is 400 m.
please mark my answer as the brainliest.
i know that the pic is a bit difficult to read as it is rotated.but u can save the pic and rotate it after opening it from the location u have saved it.if u r using ur phone then u can simply rotate ur phone itself.