The incident happened as a 60-strong gang was working on the line between Calais, France, and Folkestone, UK.
A welder was taken ill in the early hours of Sunday and carbon monoxide poisoning was later diagnosed.
Eighteen other workers have also been taken ill.
The BBC reported a French official saying they were sent to local hospitals to give them “the time needed to remove the carbon monoxide from their system.”
An investigation has been launched into the cause of the incident.