Theory:

     We placed ourselves at the open windows, and the two women stood outside on the platform, talking up to us. Satish's mother did most of the talking.

     ‘No don't jump on and off moving trains, as you did just now,' she said. 'And don't stick your heads out of the windows, and don't eat any rubbish on the way.’ She allowed me to share the benefit of her advice, as she probably didn't think my 'mother' a very capable person. She handed Satish a bag of fruit, a cricket bat and a big box of chocolates, and told him to share the food with me. Then she stood back from the window to watch how my 'mother' behaved.
Explanation:
 
Arun and Satish settled with their bags in the same compartment. They sat near the open windows. The two women standing outside the train were talking to Arun and Satish. But Satish's mother was the one who spoke a lot.

Even after the children getting onto the train, Satish's mother did not shut her mouth. She continued advising them not to jump on and off the train, put the heads out of the window, and not eat junk food from the train.

Satish's mother thought of herself as a capable person, and she started advising them. The woman who accompanied Arun did not open her mouth. The woman's silence probably made Satish's mother think Arun's mother is incapable of advising her son, and so she advised Arun too.

Satish's mother brought a bag of fruit, a cricket bat and a big box of chocolates. She gave them to Satish and asked him to share them with Arun. After giving fruits and advising them, Satish's mother moved back to watch how Arun's mother would behave to her son and what she would say to him before the train's departure.
 
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Bag of fruits
 
Meanings of the difficult words:
 
S.No
Words
Meanings
1
Rubbish Waste material, or unhealthy food
2
Capable Having the ability, fitness, or quality necessary to do or achieve a specified thing
3
Benefit An advantage or profit gained from something
Reference:
State Council of Educational Research and Training (2019). Term-3 English Standard-8. The Woman on Platform 8 - Ruskin Bond (pp. 107-114). Published by the Tamil Nadu Textbook and Educational Services Corporation.