The early morning 7:16 am service from East Grinstead to London Bridge is the most overcrowded train in the country.
It typically squeezes around 726 more people than the dozen carriages are designed for.
Roughly 1,366 people travel on the Southern service, meant for 640 passengers.
The Department for Transport says extra carriages can’t be added, but capacity will be boosted next year when the Thameslink programme is completed.