* Unions to strike in two-day waves over three months
* Railway traffic seen widely disrupted on Tuesday
* Polls show majority of French backing SNCF reform (Adds SNCF figures on striking workers, new trade unionist quote)
PARIS, April 3 (Reuters) - French rail workers launched the first in a series of nationwide strikes on Tuesday, threatening disruption over several months in the sternest challenge yet to President Emmanuel Macron's resolve to modernize Europe's second biggest economy.
French media dubbed the day "Black Tuesday." State-run rail company SNCF said 48 percent of workers needed for the network's smooth running were absent, protesting at plans to rewrite staff benefits including job guarantees and generous pensions.
Just one in four trains ran in the Paris region while only one in eight high-speed TGV trains were operating. At Gare du Nord, Paris's busiest station, platforms were so crowded that some commuters tumbled onto tracks; other stations were plunged into darkness, the lights and ticket machines switched off.
"I do understand why they're striking," said Marie Charles, a Paris commuter. "But today is my first day in a new job so I have to admit I could have done without the strike."
Outside Gare de L'Est, crowds of SNCF workers chanted slogans denouncing the government and set off flares that filled the area with smoke. Many students joined the protest.
The four main rail unions plan to strike for two days out of every five for the next three months -- a total of 36 days of disruption -- to fight a shake-up of monopoly SNCF before it is opened to competition in line with European Union rules.
The last French president to square off against rail unions over workers' benefits came off worst. The strikes of 1995 paralyzed France and forced prime minister Alain Juppe to pull the reforms -- a defeat from which he did not recover and that led to then-president Jacques Chirac dissolving the government.
The unions appear weaker now, however, and are divided over their responses to Macron's many social and economic reforms.
If the 40-year-old president triumphs it will set the tone for other reform plans, including revamping the education system and overhauling pensions. Macron has already faced down the unions to ease labor laws, making it simpler to hire and fire.
Transport Minister Elisabeth Borne urged labor unions to negotiate, telling BFM TV in an interview on Tuesday that "some of them are clearly trying to turn this into a political issue."
"The government will stand firm," Borne said.
In a battle for public support, union leaders hit back.
"The railway workers are not doing this for the fun of it," said Philippe Martinez, head of the hardline CGT union, the biggest among railway workers.
SNCF President Guillaume Pepy said the strike was likely to cost the company 20 million euros a day in lost revenue at a time when it is already highly indebted and cost-heavy.
In a bid to boost efficiency, the government wants to end rail workers' jobs-for-life, automatic annual pay rises and early retirement rights -- benefits that date back to the nationalization of the SNCF in the 1930s.
The government has painted the rail workers as enjoying outdated privileges, a notion the unions strongly reject.
"This little melody being sung of 'privileged railway workers' is intolerable," Martinez said. "The salaries aren't amazing, the work conditions are difficult, and for some it means working every other weekend."
Macron will need to keep the public on his side if he is to defy the unions and push the reforms through. An Ifop poll on Sunday showed that just over half of French people view the strikes as unjustified.
International services were also disrupted, with no trains running between France, Switzerland, Italy and Spain and the Eurostar connecting London, Paris and Brussels operating a reduced service.
Macron wants to transform SNCF, which adds 3 billion euros to its 47 billion euro debt pile each year, into a profit-maker. The unions say the debt was caused by excessive investment in France's high-speed network and accuse Macron of paving the way for privatization, which French officials deny.
The government has sought to ease tensions with assurances that the arrival of foreign competition on French rail tracks, penciled in for gradual rollout from 2020, can be delayed.
As well testing Macron's reform mettle, the rolling strike will also challenge the resolve of unions to maintain a common stance. The Communist-rooted CGT wants the industrial action to spread to other sectors but has so far seen little support.
Some energy sector workers walked out on Tuesday in protest against the planned liberalization of the power sector, but there was little impact on power output.
Air France workers also went on strike over pay on Tuesday, although the company said it expected 75 percent of flights to operate as usual. (Additional reporting by Brian Love, Emmanuel Jarry, Luke Baker and Bate Felix; Editing by Catherine Evans and Richard Lough)