The railway line is blocked in the Swindon area after police were called to reports of a person on the tracks.
All services through the town have been stopped while officers respond to the incident near Junction 16 of the M4.
Its understood the person was suffering a mental health episode and is now in the care of Wiltshire Police.
Further searches are likely to be carried out before the railway line can be reopened.
In a statement, Great Western Railway said: “Due to trespassers on the railway at Swindon all lines are blocked.
“Train services running through this station may be delayed by up to 45 minutes. Disruption is expected until 19:30 17/10.
“Apologies to customers travelling through Swindon this evening.
”One of our drivers has reported seeing a member of public trackside on the live railway near Swindon station.
”As a critical safety measure, all trains are currently on-stop whilst the police investigate.
”We apologise for any inconvenience caused to your journey with us this evening.”
