Renovation work has been halted on a series of railway carriages after asbestos was found in some of the paint.
The Federal Railways said on Monday that there was no danger to passengers.
A small area containing asbestos was discovered last Friday in the coating used on the doors and ceiling of a "Bpm 51" carriage being upgraded in the work shed in Bellinzona, in Italian-speaking Switzerland.
But the area in question is only accessible during maintenance, and asbestos fibres would only be released if it were machined in any way, the railways said, adding that the amount of asbestos found was very small and was stable.
However, 16 workers had been exposed to it. Experts are now considering how work can proceed.
In the past few months, 50 carriages of the same type have been renovated in Bellinzona and the north western town of Olten, out of a total of 180. Work in Olten has also been halted.
They are single decker carriages, normally used for extra trains laid on at rush hours or for special events.
The railways say that all rolling stock containing asbestos had been refurbished or scrapped by 2010.
The transport workers’ trades union has now called for all rolling stock of the same age to be re-examined. They also demanded that any workers who might have come into contact with asbestos should be monitored for life.
Last week Swiss industrialist Stephan Schmidheiny was sentenced to 16 years imprisonment by an Italian court which found him responsible for the deaths of thousands of workers from asbestos-related diseases.
Breathing in asbestos fibres can cause various fatal lung diseases including cancer, which normally only become apparent many years later.
swissinfo.ch and agencies