The accident happened in front of thousands of children
The accident occurred during a "test of strength" challenge to raise money for a hospital in Young, 380km (235 miles) west of the capital, Montevideo.
Contestants were pushing and pulling a train and two carriages when the vehicle gained speed and ran them over.
Local authorities have declared three days of mourning.
'State of shock'
Participants in the programme, called A Challenge to the Heart, raise funds for local charities by completing difficult tasks set by the network - in this case manoeuvring a train a certain distance down railway tracks.
The Associated Press news agency quoted Ana Portela as telling local radio station El Espectador that the train was moving when "somebody slipped and fell under the locomotive, and others were falling alongside it.
"There were shouts and somebody said 'my arm!'" Ms Portela said.
"Everybody was in a state of shock."
Graciela Baccino, a spokeswoman for Channel 10 television, said the tragedy was "deeply saddening".
She said the TV network offered its "solidarity and support" to the families involved.
About 3,000 schoolchildren were at the recording and witnessed the accident.
A runaway train killed seven people and injured eleven more today in the small town of Young, Uruguay, about 235 miles west of Montevideo. The victims were taking part in an Uruguyan reality game show, Desafio al Corazon, or "A Challenge to my Heart," where communities can raise money by performing tasks. The task in this accident was to push or pull a train with two cars attached to it down a railroad track. The train gathered too much momentum and ran down the people in front of it. 

One eyewitness, Ana Portela, said that the train was in motion when "somebody slipped and fell under the locomotive, and others were falling alongside it. There were shouts and somebody cried 'my arm!' Everybody was in a state of shock." 

Local authorities have declared three days of official mourning as Uruguayan President Tabaré Vázquez offered his deepest condolences for the victims of the crash. A spokeswoman from the channel airing the program, Mrs. Graciela Baccino stated, "Channel 10 offers its solidarity and support to the families involved in this unexpected tragedy that came, in fact, while they were trying to help others." The money raised would have gone towards a local hospital.