Student or Learner
"A person boards a train from London. One more person boards the same train and this person is following the first person. Train reaches another station and the guy who is following the first person calls his comrade who is not in the train with him that "he has been keeping an eye on the first person since they were in London." Or "he has been following that guy since they boarded the train in London."
There's no need to keep using "person" like that. It's more natural to use "someone" or "a man/woman".
A man boards a train in London. Another man is following him so he gets on the same train. When the train pulls into [name of station], the second man calls his friend/colleague/comrade/boss to confirm that he has been following the first man since they got on the train in London.
It would be much easier to read if those people had names!
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.