Island Line train services have been suspended as officers hunt the individual, who accessed the tracks at Ryde Esplanade station.
Inital indications are that the person is threatening to harm themselves.
The search is currently centered around the Monktonmead area where the tunnel exits towards Ryde St John’s.
One witness has said the man took position on the roof of an industrial unit after leaving the tunnel.
An Island Line worker is standing by at the Esplanade side of the tunnel.
UPDATE @ 19:21 – The wanted man has been successfully located and detained by Armed Response Officers and local Police units off Monkton Street.
It is thought the man was armed with a ‘bladed article’, understood to have been purchased from Poundland.
The individual is being transported to custody at Newport Police Station,
The incident has been brought to a close and Island Line train services are expected to resume shortly.