Network Rail said unions had asked for "massive" pay increases
Up to 12,000 Network Rail maintenance staff have gone on strike in a row over jobs and conditions.
The Rail Maritime and Transport union said "100%" of its members walked out for 30 hours from 1200 BST "after months of fruitless talks".
General secretary Bob Crow said the action would have a "cumulative" effect on the rail system, but Network Rail said services had not been affected.
It accused the union of making "ridiculous" demands.
Mr Crow, who visited a picket line near Brentwood in Essex, said the strike had been "100%" supported.
"The company has turned a dispute about harmonisation into an attack on jobs and conditions," he said.
"We are repeating our call for Network Rail to agree an acceptable harmonisation plan with us."
Four years ago, Network Rail took maintenance work back in-house from seven private companies.
It inherited more than 50 different sets of terms and conditions and has recently been holding talks with unions to try to harmonise them
Network Rail said the union had made unreasonable demands, such as a 34-hour working week for all maintenance staff and "massive" pay increases.
These would add more than £100m to the maintenance wage bill, the company said.
A spokesman added: "We have enough fully qualified and competent people to carry out the time-sensitive, day-to-day maintenance work over the weekend.
"Non-time-sensitive work will be rescheduled."