Abstract: A novel ball valve type purging device for a tank train ball valve purging device wherein a one-way valve is connected in an upper portion of a long vertical interconnecting pipe between adjacent cars of a string of tank cars so as to let gas pass from the long vent pipe into the top of the tank car and eliminates or minimizes the necessity to blow gas or air through the car commodity during purging of the interconnects and greatly reduces the pressure and quantities of purge gas required. The structure of the invention effectively eliminates the limitation on the length of a string of cars due to the purging requirements and the reduction of the quantity of purge gas substantially reduces the cost of purging particularly wherein inert or bottled gas such as nitrogen is utilized.