The Flying Scotsman’s inaugural run from London to York after a 10-year refit was ground to a halt by trespassers.
Network Rail said it has had to “stop all trains” on the East Coast Mainline - including the world famous steam locomotive- “because of a huge number of trespassers” keen to get a look at it.
The train is now running more than 20 minutes behind schedule.
Passengers said the famous locomotive came to a “shuddering stop” near St Neots, Cambridgeshire, and Virgin Train East Coast warned that services were being delayed due to photographers on the track.
A Network Rail spokesman updated: We are all excited to see Flying Scotsman return to our rails and we know many people have waited many years for this.
“Please stay safe and keep away from the trackside. Normal services are still running.
“The number of people beside the track is already giving us safety concerns and means we are having to slow trains down.
“Trespassing on the railway is illegal and incredibly dangerous, please help us to make today a success and stay off the railway.”