The Rail Maritime and Transport Union (RMT) is balloting its members in a row over bonus payments for staff working during the Olympics.
More strike action has been threatened ahead of, or even during, the London Olympics, with workers at four different train companies being balloted over possible industrial action.
Members of the Rail, Maritime and Transport (RMT) union at South West Trains (SWT), Greater Anglia and First Great Western will start voting this week in a row over a bonus for working during the Games.
Meanwhile, the Transport Salaried Staffs Association (TSSA) is balloting members on the West Coast line over the suspension of a union rep by Virgin Trains.
The ballot will end on July 18, less than two weeks before the start of the Games, and the TSSA warned that action could be held during the Olympics.
The RMT ballots will end on July 19.
SWT insists it has already struck a pay deal with the RMT covering the Games period and has made it clear it will not offer any more money.
But RMT General Secretary Bob Crow said it was "a scandal" that SW Trains, Greater Anglia and First Great Western had "decided to try and force their staff to work longer and harder for little or nothing in return."
London Underground says that moving control-room services to Hammersmith will increase capacity on the tube network.
A single control centre is being set up as part of a major modernisation programme. New signalling systems are also being installed on the Metropolitan, District, Hammersmith & City and Circle Lines.
The RMT union's complaints relate to the transfer of Piccadilly Line control-room services to a new centre in Hammersmith.
Transport for London plans to replace a number of separate control centres with a single one from 2017.
The Rail, Maritime and Transport Union says the move will cause job losses - and that there are also issues with rates of pay and the lack of "any meaningful offer for Service Control staff for the Olympic period."
Members of the RMT union had voted in favour of industrial action by four to one.
The row is over union demands for all Tube Lines staff to be allowed to join the Transport for London pension scheme and receive the same travel concessions as those who work for London Underground.
Workers on the Picadilly, Northern and Jubilee lines will go on strike next week in a dispute over pensions and benefits.
The Rail Maritime and Transport Union, which represents around 1,000 staff on the Tube Lines contracts, said its members would walk out from 4pm on Tuesday until the same time on Friday.
The dispute involves staff who work on maintaining and upgrading tube lines, including carrying out emergency repairs.
Union bosses said the strike would have a "widespread" impact on tube services across the network.