Trains running between Caerphilly and Cardiff were delayed after reports of a cyclist on the track near Caerphilly tunnel.
A British Transport Police (BTP) spokesman said: “BTP officers were called to the line near Caerphilly railway station on Wednesday, 12 March, following a report of a person on the tracks between the station and the Caerphilly tunnels.
“The incident was reported to BTP at 6.42am and trains in the area were initially placed on caution.
“Officers arrived and conducted a full search of the area, with all trains now on stop, but to no avail.
“I cannot emphasise enough the dangers associated with this type of behaviour. Anyone who trespasses onto the railway is taking a gamble with their own lives, as well as the lives of all the staff and passengers travelling on the trains.
“Enquiries are ongoing and anyone with information is urged to contact BTP on 0800 40 50 40 quoting log reference 48 of 12/03/2014.”