2 Answers
62It all depends on your system
If you want to analyse the man's effect on the train then man and train will be different and any force between man and train will external.
If you dont want to consider man's effect on the train then it will be inernal.
If you want to analyse the whole system in second part (which is kept inside something else then tension will be internal.
If you want to analyse each block seperately (which is generally the case) then it will be an external force
