Just when you thought it was safe to plan for the start of the 24-hour Tube, another strike looms large.
London's largest transport union RMT is balloting its 1,500 engineering staff who maintain the Jubilee, Piccadilly and Northern Lines, and it is thought that members will vote for industrial action.
It has written to Transport for London demanding answers on safety and "a fair deal" for engineers.
The result of the ballot will be announced within the fortnight, and if it goes ahead, could be timed to derail Transport for London's plans for the grand launch of the Night Tube in August.
Although the pay deal has been accepted by the vast majority of workers on the Tube, maintenance workers have been offered a different pay deal, which meant it was rejected at the start of the month.
General secretary Mick Cash said "We may have settled the dispute for LUL staff on pay and conditions but the idea that all of the outstanding issues have been resolved, not least the safety case and the Tube Lines engineers dispute, is simply not true.
"No one has seen any draft staffing rosters and those basic operational issues would be fundamental to hitting a start date this summer. There is very clearly some distance to go in terms of getting all of the components in place.
"The whole project is being constructed against a backdrop of TfL cuts and only a fool would pretend that those cuts do not have an impact on the operational model.
"RMT remains fully engaged in delivering the Night Tube and fully committed to making sure that all of our members, including Tube Lines, are properly respected and rewarded. "