The migrants were trying to make it on the train thru the Chunnel to London.
Via Daily Mail:
Chaotic scenes and 16 hour delays hit London-bound Eurostar services overnight as migrants climbed on to the roof of an 186mph train and attempted to break into train carriages.
Passengers on board one train stranded near Calais are understood to have threatened to smash the windows after a power outage left the sweltering carriage in total darkness for five hours.
The power failure also saw those on board left without air conditioning and unable to communicate with Eurostar staff before the stricken train eventually returned to Paris.
As they searched in vain for the migrants, Eurostar staff went as far as making the bizarre move of appealing to passengers to listen out for the sound of movement on the roofs of the trains.
This was an earlier attempt to storm the fences to be able to reach the Chunnel and get from France to the U.K.: