Seven of the Ballarat line’s 13 morning peak hour trains were 100 per cent full in March, sometimes from Bacchus Marsh, while seven of the 11 afternoon trains are standing room only. 

V/Line chief executive officer James Pinder told state parliament on Thursday that as quickly as its manufacturer Bombardier produces extra carriages they are filled.

“We’re adding one train a month, one train every six weeks to our fleet, but that capacity gets absorbed very quickly,” Mr Pinder said. 

PTUA Ballarat convener Ben Lever said the number of commuters using V/Line had increased so rapidly it was a victim of its own success. 

“The investment in V/Line over the last 15 years has really shown that when governments provide a decent service, people will flock to it,” Mr Lever said.

“Unfortunately it's been a victim of its own success, as the passenger numbers have grown so fast it's been hard for them to keep up with.

“The lead time in building trains is so long that both sides of politics need to maintain their commitment to a steady pipeline of investment, so we have new trains constantly rolling off the production line.”  

Mr Lever said the pain would not truly ease until the outer-suburban stations were serviced by metropolitan rains. 

“A large part of the increase in passenger numbers comes from the population growth around these outer-suburban stations, which really need to be served by electrified Metro trains,” Mr Lever said.

“The duplication of the tracks between Deer Park and Melton, which is happening in 2018-19, will lay the groundwork for this electrification, but the government claims that there won't be enough capacity in the CBD until the Melbourne Metro tunnel is finished in 2026.”

Western Victoria MP Joshua Morris urged the state government to continue to invest in the network to ensure regional people did not lose out.

“This issues with congestion in the outer western suburbs are having a detrimental effect on the Ballarat service and everything that can be done must be done.” 

A spokesman for Minister for Public Transport Jacinta Allen said the number of services had matched the increases in patronage on the line.  15 new carriages to Ballarat line services since mid-2015.

"In just over two years we've introduced more than 60 extra services and 15 new carriages for Ballarat every week -- creating space for thousands of extra passengers every single day,” Ms Allen said.