Workers at four train companies are to be balloted for strikes in separate disputes which threaten action during the Olympics.
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 said it was balloting more than 500 of its members on the West Coast line for strike action 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.
The RMT said it has concluded "substantial" pay deals covering the Olympics at First Capital Connect, C2C, Chiltern, East Midlands Trains and East Coast following earlier agreements for a bonus at Heathrow Express, Virgin, Network Rail, Docklands Light Railway and London Underground.
General secretary Bob Crow said: "With deals now concluded that recognise the massive additional effort of the workforce on the vast majority of transport operators, it is a scandal that SW Trains, Greater Anglia and First Great Western have decided to try and force their staff to work longer and harder for little or nothing in return.
"We all want the Olympics to be a success but we cannot have fairness on some train companies and a slap round the face on others.
"It is rail staff who will be sticking in the extra work required to deliver the transport challenge of the Olympics, these train companies will be making a killing from the extra passengers and services and they should have the decency to match the best practice on payments across the rest of the industry."
A Virgin Trains spokesman said: "Our staff are really enthusiastic about playing a major role in helping thousands of spectators to the various Games venues.
"For the extra work involved they will receive a £500 bonus. We are encouraging all TSSA members to understand the issues in this case, and to make sure they cast their vote."