The numerical words "majority" can be plural or singular. When they refer to a group of people they are singular. Else you need to see the context. If you are using individual parts of the totality use plural - source Manhattan GMAT
eg. The majority IS (singular) opposed to the decision
In the ones you cite
The majority of students is/are smart.
I would go with "are" The majority of students is plURAL
The majority of physics students is/are smart.
Same as above
Incorrect: The majority of railway commuters reads or listens to music while traveling.
Again it should be plural hence
Correct: The majority of railway commuters read or listen to music while traveling.
A majority of, also takes plural verb.
eg. A majority of students AGREE with ....
So in your case
A majority of students is/are smart.
although idiomatically, I would prefer The majority of students
A majority of railway commuters READ or LISTEN to music while traveling.