The Automatic Train Operation of a metro line is not so difficult to implement: almost no junction or crossing between lines, rolling stock homogeneity, double track operation, relatively low speed, very simple station track layout, mostly rubber-mounted, and specific site continuously fenced with no level crossing. Tunnels and platform screen doors complete the picture to prevent almost totally the presence of an obstacle or a person on the tracks. But how can Automatic Train Operation for a metro line be transposed into a long distance train line or further into a country wide network? This paper reminds firstly the different Grades of Automation. Then, it shows Automatic Train Operation used in conjunction with the Communication Based Traffic Control that drives Automatic Train Operation of a metro line. Some information about Automatic or Automatic-like Train Operation Systems are also given in a traditional rail network context and what it is possible to do right now. Finally, the paper presents some alternatives to face challenges met in a Semi-Automatic Train Operation context and in an Unattended Train Operation context. To resume, the actual use of the Packet 44 of the European Train Control System is sufficient for Semi-Automatic Train Operations. Nevertheless, a speed information should complete the actual definition of timings points to obtain a more efficient Automatic Train Operation system. Unattended Train Operation is thinkable in the future, namely with the help of video/infrared cameras in front of the train. Images processing and remote control will then deal with risks and degraded situations.