A man was found with serious injuries in the subway train station of Tonypandy, Rhondda.
Police were called at 1am on Friday after the injured man was found and he was taken to hospital with serious injuries.
The man who is in his 30s and is from the local area is currently in hospital where his condition has been described as critical. At this stage his injuries are unexplained.
Part of the area has been cordoned off so investigations can be carried out at the scene. Rail passengers using the station are being told they need to use the Trealaw entrance.
One rail passenger said there was a "lot of police activity" at the station this morning.
Anyone with information should contact South Wales Police Rhondda Valleys.