The Metropolitan Line has reopened after it was forced to close on Saturday due to the “pingdemic”.
Control roof staff were unable to turn up for work after being told to self-isolate by the app in teh morning.
No services ran on the Metropolitan line, which runs between Aldgate and Amersham, Buckinghamshire, as staff had been contacted by NHS Test and Trace.
The Piccadilly and District lines were also affected, but have also reopened.
The Government has been warned over a “surge” in workers and medics being forced into self-isolation over coronavirus contacts when most restrictions end in England on Monday.
Transport unions have said there will be “dire consequences” next week when staff are “pinged” as the level of infections rise.
Rail, Maritime and Transport union (RMT) general secretary Mick Lynch previously said that Monday “will see a surge in workers pinged with a self-isolation instruction next week”.
“Even at this late stage, the Government, the train operators and the bus companies should issue a clear, legally backed instruction that levels up the rest of the UK to the safety standards that will remain in force in Wales and Scotland,” he said.
Elsewhere in the country, passengers were warned of alterations on some Northern routes this weekend due to a number of front-line staff either testing positive for Covid-19 or having to self-isolate.