It was not until the following day he knew his son - one of 83 dead  - had perished in the Granville train disaster.

"He'll always be my son," Mr Pederson said.

Mr Pederson, 103, joined other families of loved ones and survivors, along with the Prime Minister, Premier and Governor and emergency service personnel, in Granville on Wednesday to commemorate the 40th anniversary of Australia's worst rail disaster.

The day commenced shortly after 8am with the ringing of bells and the reading out of each of the victims' names at the Memorial Wall near the Bold Street bridge, the original of which fell onto the derailed train in 1977.

This was followed by a Christian memorial service at St Marks Anglican Church, which was run by the families of those lost and survivors of the accident.

Addressing the congregation was secretary of the Granville Train Disaster Association and daughter of victim Bryan Knight, Meredith Knight, who said she and her family had been devastated but "life goes on".

"There is no closure," said Barry Gobbe, the first ambulance officer on the scene in 1977.

A parliamentary apology will take place in February with many of the survivors upset at the lack of government assistance over the many years since the disaster.

Eighty-three candles were lit inside the church with Reverend Peter Hayward telling the mourners that the roses which have been synonymous with the occasion are a "reminder that God is the one we can look to for help".

With the road closed, the nearly 300 strong procession accompanied by a lone bagpiper made their way to site of the tragedy to pay their respects.

Dignitaries then lined the top of the railway bridge with Prime Minister Malcolm Turnbull the first the throw a rose onto the tracks.

"Today is a day of grief and remembrance and a day to comfort those who lost their loved ones," said Mr Turnbull, who also acknowledged the heroism of the emergency service personnel.

Premier Mike Baird, State Opposition Leader Luke Foley and Cumberland Council Administrator Viv May were some of the speakers at the Memorial Wall, all sharing the grief of the families and victims as well as thanking the emergency services that attended on that fateful day.

Acknowledging that an apology was "long overdue to everyone who has lost a loved one", Mr Baird hoped that the victims would never be forgotten.

More than  50 wreaths, including from NSW Governor David Hurley and Sydney Trains' CEO Howard Collins, were laid alongside the Memorial Wall which has recently received a renovation with a spelling correction of 13 of the victims' names.