There are a certain of cases in modern practice where operating at higher at higher speeds can could the brake force above a certain speed would be insufficient to stop the train within the required distance. Some cases arise because the brake force to weight ratio of a locomotive of typically 80 to 120 tonnes in weight is often less than that of passenger vehicle in the 40 tonne range; and it may be that locomotives running by themselves require several coaches to be attached if the train is to run at its maximum permitted speed with a suitable brake force to weight ratio. Another issue that can arise is when a locomotive is hauling a set of coaches, typically a multiple unit, where it is unable to work the brakes of the unit. In this case additional braking vehicles may be attached or the train may need to run at reduced speed.