"I always said that we had priorities on which we could not give ground but that there were issues that were up for negotiation," he said in Lisbon, the Portuguese capital, after an EU summit.
"Firmness is also about being able to listen to the other side. That's what we are doing." Bitter split
Other unions were angry about the move.
They had hoped to keep pressure on the government with a united front against plans to end the right of a minority of state workers to retire with less than 40 years of paid-up contributions.
Jean-Luc Gironde, a spokesman for the Force Ouvriere union, accused the FGAAC of "saving its own skin".
Gironde said: "Worker solidarity, tell me where it is?"
All eight rail branches took part in Thursday's strike and they are meeting again on Friday and Monday to decide whether to carry on striking.
Jean-Michel Namy, the FGAAC spokesman, labelled criticism from other unions as "theatrics", and said as far as his members were concerned any question of further strikes was "over".
Bernard Thibault, head of the General Confederation of Labour (CGT) union has said future strikes would only be effective if unions were united.Strong support
The government wants to put state employees paying into so-called "special regimes" on an equal footing with civil servants and private sector workers, increasing their contribution period from 37.5 years to 40 years.
However, it had indicated it would still be willing to give preferential treatment to a much smaller group of workers based on an assessment of the strains of their job.
Under the terms of the FGAAC accord, drivers will still be able to retire at 50, but on a lower pension.
Meanwhile, Xavier Bertrand, the labour minister, will meet the main unions involved in Thursday's strike next week. The special funds were introduced after the second world war, mainly for workers in physically demanding jobs, but they are running deficits that will cost the taxpayer an estimated $7bn this year.
Polls show strong support for the government's changes.
Lengthy protests have crippled France in the past, proving fatal 12 years ago to a conservative government which was forced into a humiliating climb-down over similar pension changes.
But an Ifop poll on Wednesday showed 82 per cent of people backed reform of the "special regimes", and 61 per cent opposed the strike.
The senior members of the Socialist opposition and even unions agree that state pensions must be reformed, but argue that workers should not bear the brunt of any changes.