Plan to end trains dumping waste on Scotland's railways
Retention tanks are set be fitted to high-speed trains to stop the dumping of human waste onto railway tracks.
The practice of dumping sewage on the railways was ended by ScotRail in 2017.
However, it was reintroduced last year after delays to a new fleet of refurbished trains forced ScotRail to hire carriages without toilet waste tanks.
The rail operator said it was "doing everything" it could to meet a UK-wide ban on dumping the waste by 2020.
ScotRail was meant to have received 26 refurbished high-speed trains, with waste tanks fitted, for routes linking Scotland's seven cities from rail firm Wabtec by December last year.
So far, the firm has only delivered five of these models - which date back to the 1970s but have all been renovated - and ScotRail has hired 14 'classic trains, without waste retention tanks fitted, to make up some of the shortfall.
Track operator Network Rail has committed to end the practice of effluent discharge from passenger train toilets by the start of 2020 and now ScotRail has advertised for firms to bid for the work to fit retention tanks to its 'classic' trains.
Mick Hogg, the RMT union's regional organiser in Scotland, said: "Every day our members are having to deal with the consequences of this broken promise of ending this disgusting practice for good.
"If it was a bus company dropping human waste on to the streets of Edinburgh or Glasgow, there's no way it would have taken this long to act.
"Fitting the tanks is a welcome step but it has taken too long, and I am sceptical that they will get this work done by the end of the year and meet the Network Rail target."
Research by industry regulator, the Office of Road and Rail, found that the risk of infection to railway workers from the waste was low.
The ScotRail advert asks for firms to submit proposals for waste retention tanks to be fitted between August and December this year.
No firm timetable
The rail operator said it was not possible at this stage to say how much the work would cost.
There is also no firm timetable on when the remainder of the refurbished trains from Wabtec will be delivered but ScotRail has previously indicated it will seek compensation for the delays.
A ScotRail spokesman added: "We're exploring the option of fitting toilet retention tanks to our classic high-speed trains. This is due to the late delivery of the trains from our supplier.
"We are aware of the UK-wide target and we are doing everything we can as a business to meet it."