A minor ammonia (NH3) leak occurred on a pressurised 48-tonne railcar sitting idle at an industrial site. The NH3 concentration in contact with the leak on a manhole flange was on the order of 30 ppm and situated less than the 1-m detection threshold. Since the site’s storage tanks were full, railcar contents could not be immediately transferred; consequently, the car was moved under windy conditions and additional measures had to be taken to avoid aggravating the situation: installation of a Gore-Tex joint and collar on the flange, organisation of inspection rounds, deployment of water curtains, information conveyed to first responders, Inspection Office staff and the neighbouring company. Railcar contents could be transferred 1.5 days later; the total quantity of NH3 released into the atmosphere was evaluated at 1 kg. The site operator was required to submit a report to Inspection staff on the origin, causes and outcome of the incident, in addition to specifying the measures adopted to reduce the probability of its recurrence.