Aslef, which represents 1100 drivers in Scotland, said it was increasingly frustrated at the lack of information from ScotRail about the extra duties for staff created by the Games.
Kevin Lindsay, Aslef Scottish general secretary, said no-one wanted to "hold the Games to ransom", but warned: "If we don't reach a settlement then ScotRail will have difficulty running their services."
Aslef argues Glasgow 2014 is "Scotland's Olympics" and all train drivers should receive a one-off payment for extra work as spectators will travel from around the country to Glasgow, and extra services will mean knock-on effects across the rail network.
Up to 1500 conductors and ticket inspectors represented by the RMT, around 300 engineers represented by Unite, and up to 800 station staff belonging to the TSSA are also seeking bonus payments.
Based on the Olympics, these are expected to be a few hundred pounds per person.
However, a lack of information filtering down from Glasgow 2014 and the Government agency Transport Scotland means train operator ScotRail has yet to finalise the extra services that will be required.
"This in turn has left the unions unable to negotiate bonus payments.
One senior union source said "amateurs were running the show" at Glasgow 2014, adding: "The London Underground timetable for the Olympics was agreed four years in advance.
"Here we are four months away and we still don't know what trains are running. They don't know their arse from their elbow.
Lindsay told the Sunday Herald: "We are looking for a bonus payment for all staff involved.
"If we don't reach a settlement then ScotRail will have difficulty running their services during the Games. The last thing that we want is to hold Scotland's Commonwealth Games to ransom.
"All we are looking for is a fair share.
"We would have hoped to have put this to bed months ago.
"But until we get proper clarity from ScotRail we can't sit down to negotiate [a final figure]. This looks like it will go down to the wire and that's not a good place for any of us to be."
The other unions seeking bonus payments said there was mounting frustration at the lack of information needed for staff planning.
Talks have been under way since late last year, with a fourth round held on Thursday.
Michael Hogg, RMT regional organiser, said: "We're still unclear about detail.
"We've had four meetings with ScotRail and don't seem to be making much progress.
"It's got to a ridiculous stage."
Doug Blundell, TSSA regional organiser, said he had worked on a bonus deal for London Underground staff during the Olympics and that was agreed "years in advance".
He said: "Three months away and we still don't know what the detail is - where the extra work will be and what is needed.
"They seem blasé about it. It's worrying."
Eddie Duffy, Unite transport officer for Scotland, added: "The problem we have is there's no definite information.
Transport Scotland said it was close to agreeing an overall funding package with ScotRail for extra services and carriages during the Games fortnight.
"As is normal, the agreed plan for the additional services needs to be formally approved by Network Rail," a spokesperson said.
"While industrial relations are a matter for ScotRail and the unions, Transport Scotland is encouraged all parties are continuing negotiations to resolve this issue."
A ScotRail spokesperson said: "We are making progress and are due to meet again in a couple of weeks with each of the trade unions to take matters forward."
A spokesperson for Glasgow 2014 said: "We are working collaboratively with a wide range of national and local delivery partners to ensure a cohesive, comprehensive and efficient transport network for Glasgow and Scotland's biggest-ever sporting and cultural celebrations.
"We have positive working relationships across our networks and are pleased with the integrated delivery plans emerging from these collaborations."