Automatic control system model of second train locomotive at virtual assessment as well as control quality analysis approaches have been considered. The controlled value represents being calculated in train traffic process the distance between “tail” of a first train and “head” of a second one, providing traffic safety at emergency braking of a first train. This distance is called “virtual replacer length”. Formalization has been introduced allowing to account for synchronization absence at transfer of information on the coordinate and speed of a first locomotive and the control digital system work cycle of a second one. Speed and coordinate measurement errors have been considered which’re connected with lagging at information transfer and possibility of failing in decoding of received by radio channel information by second locomotive receiver. At virtual length assessment, various ways of its upper assessment have been considered, the way has been chosen, taking into account motion speeds and the coordinates of both trains, emergency braking path of a first train, the possibility of emergency braking of a first train at the beginning of the cycle of information transfer, measurement methodological and instrumental errors. Digital proportional integral differentiating control law has been considered as an operator, transforming mismatches at the choice of a control of a second locomotive. Automatic control system model has been developed which relates to impulse system class and which feature is the calculation of control action in train traffic process. The parametric synthesis of automatic control system is recommended to be held by simulation modelling methods. Control quality criteria have been chosen.