Answer: It remains the same
Explanation:
The Conservation of Momentum principle establishes that the initial momentum must be equal to the final momentum :
(1)
In the case of the train, its initial momentum is:
(2)
Where is the mass of the train and is the initial velocity of the train
And, its final momentum is:
(3)
Where:
since we are told the mass of the train is doubled
since we are told the velocity of the train goes to half
Hence:
(4)
(5)
(6)
This means the momentum of the train remains the same.