Train drivers' union Aslef called for an urgent review of security after a night Tube driver was kicked and sprayed with paint by vandals.
The driver, who was returning a Jubilee line train to the depot at Stanmore, had responded to an automatic alarm signalling the opening of a rear door.
Transport for London (TfL) condemned the attack and said it was reviewing security arrangements at the site.
The British Transport Police said it would increase patrols around the area.
'One in a series'
Officers were called to Stanmore station shortly after 04:00 GMT on Sunday.
Police believe the driver disturbed a group of individuals who were "vandalising a train carriage".
Aslef said he was confronted by a group of men who took his radio and sprayed him with paint. The union said he was also "tripped and kicked".
Finn Brennan, Aslef's organiser on London Underground, said the assault was "one in a series" of attacks on night Tube drivers since the new service was introduced at the end of 2016.
"This incident highlights concerns about the security of depots where trains are left overnight," he said.
"When the service was introduced, we were promised that additional staff and police resources would be in place to make sure it would be safe to operate.
"Instead, staff have been left alone and vulnerable in dangerous situations."
British Transport Police said it would increase patrols on the Jubilee line around Stanmore station "over the coming weeks". However, it could not comment on whether there had been an increase in attacks on night Tube drivers in recent months.
Steve Burton, TfL's director of enforcement, described the attack as "senseless and cowardly".
"We are now doing everything in our power to work with the police to bring the attackers to justice," he said.