Britain’s rail network was snarled by another mass strike Friday, with the biggest train drivers’ union warning that more industrial action could be announced if talks next week don’t lead to a breakthrough.

The labor group Aslef said more than 12,000 drivers were walking out for the second day this week, shutting down services on 15 of the UK’s main train lines. Major stations in the capital such as London Bridge and Victoria were closed.

Mick Whelan, Aslef’s general secretary, said Friday that no progress had been made in talks. However, speaking on LBC radio he added: “We want a resolution, my people don’t want to be losing money and they don’t want to be standing out in the cold.”

Talks between Aslef and train company representatives are due to resume on Tuesday. Whelan has warned that more rail strikes could be held before the end of the month if a deal isn’t reached.

“We will sit down based upon where we are and whether the talks are being successful or not, whether we believe there’s going to be any movement, and then we’ll make a decision accordingly,” he said Wednesday from a picket line outside Euston station.

“I imagine if there is no movement, there will be more dates.”

Aslef rejected an offer in January of a 4% backdated raise for last year and a further 4% this year, arguing that it amounted to a real-terms pay cut alongside changes to working practices. Rail bosses said it would have put the average driver on an annual base salary of nearly £65,000 ($79,900.)

Friday’s action follows a strike on Wednesday, when as many as half a million workers in many parts of the public sector, including teachers and Border Force officials, protested against below-inflation pay deals.

Whelan denied unions were working in league with each other to create days of widespread action. “If it’d been coordinated I think there’d have been more than 500,000 — there might have been a million or maybe two,” he said Friday.