Trapped children had to be freed from underneath a miniature railway after it came off the tracks.
One witness described hearing a crash and banging during the incident in Watford's Cassiobury Park on Tuesday afternoon.
A spokesman for Watford Borough Council said no one was seriously injured in the incident.
A report into the accident is set to be submitted to the Health and Safety Executive on Wednesday, he added.
Daniel Reichmann, 26, from Enfield, was on board the train with his five-year-old daughter and several other members of his family.
He said: "We were going along on it for about five minutes, then we heard this crash and banging and some screaming.
"And then we realised that one of the carriages had derailed and two or three behind had gone into the back of it.
"A few children fell out. I'm a trained paramedic, so I jumped out and went to help.
"Two or three children were trapped underneath the carriages.
"It's not as if it was a heavy locomotive, but they were screaming and crying."
He added: "Everyone was a bit shocked - it's not the sort of thing you expect to happen on a children's ride."
A spokesman for the council said: “The Cassiobury Park children’s train derailed on Tuesday afternoon.
"Watford Borough Council Park Rangers attended the scene.
"Nobody was seriously injured and the ambulance service was not called to the area. The children's train operator will be submitting a report on Wednesday, setting out what has happened to the Health and Safety Executive and they will determine if further investigation and action is needed.
"The children's train operator is an independent business that leases the land from the council.”
The operator could not immediately be reached for comment.
BBC News quoted Jeff Price, described as owning the railway, as saying one woman was taken by her family to hospital for a cut on the head and there were some "grazed knees".
