The operation of a bidirectional single track line has obvious problems, the most serious of which is the possibility of two trains traversing the line travelling towards each other, each driver unaware that the other is using the line. The simplest method of controlling such a line is to have only one loco operational, known as One Engine in Steam and later One Train Working (OTW). The token system was developed in Britain in the 19th century, to facilitate safe working of single-line railways. If a branch line is a dead end with a simple shuttle train service, then a single token is sufficient. The driver of any train entering the branch line (or occupying any part of it) must be in possession of the token, and no collision with another train is possible.