The electrical road system (ERS) truck formation operation is the most common working condition of this new road freight transportation system. Because two neighbouring trucks with unfavourable distance can interrupt the current collection process, the favourable distance must be obtained to prevent it. In this work, the influence of truck formation operation on pantograph-catenary interaction dynamics in ERS is first investigated to determine this favourable distance. Based on the structure of the ERS test line, an ERS pantograph-catenary model with double pantograph is formulated, and the measurement data from the ERS test line are used to validate the present model and provide truck vibration inputs. Through the present model, the influence of double pantograph distance on its dynamic behavior is investigated. It is found that the pantograph distance without truck vibration has little influence on pantograph-catenary interaction, but the truck vibration can heavily influence the same within a certain distance. This influence is dominated by the second frequency peak of the truck vibration, and its change in the panhead can be used to monitor the distance. Based on this, a method is finally given to determine the closest favorable distance between neighboring trucks in ERS.