Hundreds of staff have been balloted by trade union Unite in a dispute over pay and conditions.
Up to 150 engineers at First Group Glasgow have rejected a 2014 pay offer of 0.5%with a further 0.4% on the basis of accepting reduced sick pay allowance.
Meanwhile, more than 270 subway staff have been told they will receive no additional payment for putting on what operator SPT describes as an "enhanced service" during the Commonwealth Games.
A ballot for industrial action will run until June 30 with the results expected shortly after.
Unite regional industrial officer Eddie Duffy said: "We'll be recommending strike action during the Games to highlight the fact these people have gone six years without an inflation-proof pay rise. If we can't highlight it now, we never can."
Mr Duffy blamed both First and SPT for the breakdown in industrial relations. He said: "Unfortunately we've been brought to this point by an absolute failure of management at First Group and SPT to properly consult with and recognise the value of their employees.
"First Group are actually imposing a real-terms wage cut on its Glasgow engineers. SPT have simply gone AWOL, offering workers absolutely nothing for the increased demands they will have to undertake.
"Frankly, with the world coming to town next month both companies have shown total contempt for their workers and are trying to milk what should be a memorable event for Glasgow on the cheap."
An SPT spokeswoman said: "It is simply not true that staff will lose out on pay. SPT has made it clear that any subway staff requested to work additional hours will be paid in line with their current terms and conditions. It is incredibly disappointing that Unite is threatening to jeopardise the transport arrangements during the largest multi-sport event Scotland has ever hosted."
Managing director of First Glasgow, Fiona Kerr, said the threat of strike action left her "surprised and disappointed".
She said: "We have serious concerns that union officials have failed to properly consult with their members. Despite receiving two formal offers to date, the union has not conducted any consultative ballots to allow members to express their views."