The RMT has said it is appalled that high-speed trains being brought into service in Scotland will dump human waste onto train tracks.
The rail union said the practice, which had ceased after a deal with ScotRail in 2017, would result in its members being "sprayed with human waste".
The trains are being used as an interim measure pending the arrival of a new fleet of refurbished trains.
ScotRail described the situation as "far from ideal".
The rail operator said that of 10 newly-refurbished trains due to come into service in December, only one had arrived from contractor Wabtec.
It said the delay in the order meant "classic" high-speed trains that did not have waste tanks would operate on timetabled routes linking Scotland's seven cities instead.
'Filthy and disgusting'
Currently the only route that has a train fitted with a waste tank is the service running between Aberdeen and Edinburgh.
A deal between the RMT, Transport Scotland and ScotRail had ended the dumping of human waste onto rail tracks in December last year.
RMT general secretary Mick Cash said: "Our members are rightly appalled that ScotRail have sought to rip-up a clear agreement to end the filthy and disgusting practice of dumping human excrement on Scotland's railways.
"This disgraceful and retrograde step must be halted now and the union will take whatever action is required to hold ScotRail to their agreement with the union.
"One minute ScotRail are bragging about their aim to operate the most modern railway in the world and the next they are back to spraying track workers with human waste."
A ScotRail spokesman acknowledged that it was likely to be some time before newly refurbished trains with human waste tanks would be ready.
In a statement, the company said: "We are working flat out to get more upgraded trains into service as quickly as possible, but our supplier, Wabtec, is behind schedule and has let us down.
"To ensure we can deliver services for our customers we will introduce classic high-speed trains until enough upgraded Inter7City trains are available.
"These classic trains will not have retention tanks, which is far from ideal. We are doing everything we can to mitigate this as soon as possible."
A Transport Scotland spokeswoman said it completely sympathised with unions over the issue.
She added: "The Scottish government has directly funded previous installation programmes to eradicate this practice across ScotRail fleets.
"We understand this interim measure is regrettable, but necessary to support more services and jobs across the country while retaining passenger services between our key seven cities.
"Wabtec must deliver their work as soon as possible, so passengers and staff can benefit from these improvements."
A spokeswoman for Wabtec said the company did not wish to comment on the issue.