Theory:

     ‘What do you mean?’ asked Sir Francis.
 
     ‘I mean to say that the train isn’t going to go any further.’
 
     The general at once stepped out, while Phileas Fogg calmly followed him, and they proceeded together to the conductor.
 
     ‘Where are we?’ asked Sir Francis.
 
     ‘At the hamlet of Kholby.’
 
     ‘Do we stop here?’
 
     ‘Certainly. The railway isn’t finished.’
 
     ‘What! not finished?’
 
     ‘No. There’s still a matter of fifty miles to be laid from here to Allahabad, where the line begins again.’
Explanation:
 
Sir Francis was confused with the answer of Passepartout. He thought to reach Calcutta on the same train. Passepartout confirmed to him that the train could not continue further. Passepartout's reply aroused Francis' anger. He got down from the train to meet the conductor. Phileas Fogg calmly followed him. Sir Francis directly went to the conductor and demanded an explanation. Firstly, he questioned him as 'Where are we?'. The conductor replied to him as it was the village of Kholby. The train stops there, as it couldn't continue since the railway route was unfinished from that place to Allahabad. The general was not convinced with this answer and questioned the conductor again: ' What! not finished?'

The conductor clearly explained that there are fifty kilometres from Kholby to Allahabad; since the railway built on those fifty kilometres from Kholby to Allahabad was under process, the train stopped in Kholby. The train facility to Calcutta will again begin in Allahabad.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Hamlet
A small village
 2.
Further
Far or to a greater distance
3.
Laid
Put down or set in for a particular use
4.
Proceed
To start or continue doing something
Reference:
State Council of Educational Research and Training (2019). Term-3 English Standard-7. A Journey by Train by Jules Verne (pp. 79-85). Published by the Tamil Nadu Textbook and Educational Services Corporation.