Maya..
That time .. it was partly impossible to get that train for shooting...
BUT PM 'Indira Gandhi jee and CM of WB Dr. Bidhan Chandra Ray' took care of it...
Thats the Reason...

Apu and Durga share an affectionate brother-sister relationship. Durga, as the elder sister, cares for Apu with motherly affection, although she does not spare any opportunity to tease him. They share the simple joys of sitting quietly under a tree, running after the candy man who passes by ringing bells, viewing pictures in a bioscope shown by a travelling vendor, and watching a play by a travelling troupe of actors. In the evenings, they can hear the whistles of trains far away. One day they run away from home to catch a glimpse of the train. The scene depicting Apu and Durga running through Kaash fields to see the train is one of the memorable sequences in the film. While playing one day in the bushes, they discover their Aunt Indir lying dead there.

And commercials can intrude anywhere irrespective of any emotions and reasons

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