A strike by 12,000 rail maintenance workers has not disrupted passenger services, Network Rail has said.
It said it had "enough contingency staff" to cope, but the RMT union said the strike would have a "cumulative effect" on services.
Workers are striking following a dispute over the harmonisation of pay and conditions.
Members of the RMT union are taking part in the 18-hour walkout, which is due to end at 1800 BST on Sunday.
A spokesman for NR said: "As predicted, it is having absolutely no impact on passenger services."
He said only about 2,000 of those striking would have been working on Saturday adding that "of that figure, only a third would have been involved in time-sensitive duties".
'Solid' strike
The spokesman added that NR has "more than enough contingency staff" to combat the effects of the industrial action.
But the RMT said it was expecting the strike to be "as solid as our last strike in June which wiped out Network Rail's planned weekend maintenance".
Earlier this week the union accused NR chiefs of "scuppering" the progress being made on pay talks.
However, another union involved in the same dispute, Unite, will not be taking similar action and voiced concerns about the action being taken by RMT members.
In a letter, which has been released by NR, Unite said it was "loathe to criticise another union" but could not understand why RMT members were being asked to take strike action when negotiations were "clearly still ongoing".
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