High Wycombe – police were called to an emergency incident at the station
Eywitnesses said a train was evacuated as it waited at a platform.
Armed police told passengers to move "as far away as possible" as they surrounded the scene.
A National Rail spokesman said: "Due to the police dealing with an incident near the railway at High Wycombe, there are delays of up to 60 minutes to journeys between West Ruislip and Princes Risborough. This is expected to continue until 17.00."
Rail operator Chiltern Railways said: "Both lines through High Wycombe are currently blocked due to the emergency services dealing with an incident."
British Transport Police said officers were called to support Thames Valley Police with the incident which is "not suspicious".
Security is heightened in Britain following a suicide bomb in Manchester last week which killed 22. In the aftermath of the attack, British police put armed officers on some trains for the first time.
More to follow…
HIGH WYCOMBE TRAIN STATION. AT STANDSTILL.ARMED FORCES. ….������������ pic.twitter.com/BtcDBlrd5o
— ali (@m8tr1x) May 30, 2017