Controller of site safety

This article is about rail safety. For other uses of COSS, see Commercial open source software.

Controller of site safety or COSS is a person qualified with Network Rail to ensure safe practice for work occurring on or near rail infrastructure. Their role is to set up a safe system of work which looks after the interests of a group of people who need to go on or near the line. There are two categories of Zones that a COSS can set up, a "Green Zone" or a "Red Zone".

Green zone working means all or some lines are blocked to train movements, or work is being carried out behind a fence or more than 2 Meters from the nearest running rail. This is the safest way of working, although many incidents still happen within Green Zones.

Red Zone which means the lines are open to train movements, this is seen as unsafe and is supposed to be very much avoided in the rail industry, and only worked when green zone circumstances are unavailable.

The COSS is responsible for the safety of the entire group and is subject to prosecution should someone be killed or injured by their negligence. To become a COSS someone must have served a suitable period of time on the railways and undertake a 5 day course. This is then followed by a period of mentoring by someone experienced as a COSS and then numerous assessments to ensure that the subject is competent to undertake their role safely and effectively.

A COSS is distinguishable on site by wearing a blue armlet with white lettering on with the word COSS.

Previously, the COSS was known as the Person In Charge Of Work, or PICOW.