Tube workers formed picket lines outside stations in a day of strike action against compulsory staff transfers.
Members from the Rickmansworth Station Group (RSG) of the National Union of Rail, Maritime and Transport Workers (RMT), went on strike on Thursday at Northwood, Northwood Hills and Moor Park stations.
About 60 union members demonstrated at the stations and others at the north end of the Metropolitan line to protest against compulsory transfers.
Oliver New, a member of the union's executive council, said: "We are in dispute because there are five members of the RMT who are being told they have to transfer from this group of stations to central London.
"The issue is we work very extreme shifts and an early start at Paddington would be 5am. They have bought and rented places in this area, so to be able to get there before the trains are running they would have to leave home at about two in the morning."
The union believes this is in breech of all London Underground's policies on transfers, and say the five staff members were sent to the RSG area on permanent postings.
Mr New said: "Almost every RMT member in the group went on strike and almost everyone has supported it."
One picketer at Northwood station said: "This is a local dispute but we have a lot of people coming out. They are fired up because it's close to home.
"We have got to fight for our colleagues because it could be us, and the local management here have been good about it."
A Transport for London spokesperson said: "Five members of staff are being transferred from parts of the network where there are enough staff, to stations on the network which have vacancies and where staff are needed.
"The transfer is within the terms and conditions of London Underground employment.
"We provide transport for staff working shifts which commence and finish outside of Tube operating hours."
Union leaders hope the strike will get senior managers at London Underground to sit down with union representatives to find an acceptable solution to the dispute.