More than half of Melbourne's railway lines are consistently overcrowded in the morning peak, with some trains carrying hundreds more passengers than they are meant to.

In the worst case, on the Cranbourne and Pakenham lines, trains built to carry just under 800 passengers are crammed with more than 1200 people before they reach the city, data shows.

There is 'overloading' on all train lines servicing the north and western suburbs. Credit:Michael Clayton-Jones

The data, obtained through freedom of information laws by the Victorian Greens, reveals the system is failing to cope with passenger growth.

Between May 2014 and May 2015, an extra 4500 commuters joined the Dandenong rail corridor crush in the morning over a four-day survey period, but just one extra morning peak service a day was added to the timetable.