A union campaigning for a swift end to train toilets dropping sewage on tracks is particularly concerned about workers' health at a Leeds train depot.
Unite is concerned for workers at the Leeds Neville Hill depot, who have to clean sewage left stuck to undercarriages of trains.
The union is considering balloting for industrial action over the issue.
East Midlands Railway, which runs the east Leeds depot, said it was working to have all its trains fitted with controlled emissions toilets - which empty waste into tanks - by the end of 2020.
But Unite believes railway operators are not moving quickly enough to end the practice.
Unless there are specialised washing facilities, maintenance workers have to clean the sewage off before repair work and inspections can be undertaken.
Unite said it is particular issue at the Neville Hill maintenance depot in Leeds, where workers maintain trains from both East Midlands Railways and London North Eastern Railways (LNER).
Both companies operate trains which allow sewage to drop onto tracks and their trains.
Harish Patel, Unite national officer for rail, said: “Our members are being forced to work in disgusting conditions, which is directly endangering their health.
“The underside of trains are dirtier than sewage plants.
“The only way to protect workers’ health is to have the trains cleaned in a separate purpose built facility. This is being undertaken in many depots, where this is not happening we are advising our members the health risk is unacceptable.
“Unless immediate action is taken to introduce specialised washing facilities, Unite will begin the process of balloting its members for industrial action.
“Delays and excuses are simply not acceptable.”
An East Midlands Railway spokesperson, said: "The vast majority of our trains are already fitted with Controlled Emissions Toilets and over the last 18 months, we have also retro-fitted tanks to more of our regional fleet of trains as well.
"We completely support the drive by Network Rail to remove all trains without controlled emissions toilets by the end of 2023 and are already working towards having all our trains fitted by the end of 2020.
"This work is part of our £600 million investment plan to completely replace our entire train fleet with either brand new or fully refurbished."
London North Eastern Railway (LNER), whose trains are also maintained at Neville Hill, said only its diesel trains, which form a small part of its fleet, were not fitted with retention tanks for toilets and would be removed from service by January 1.