This paper introduces an infrastructure free multi-sensor based positioning solution for rail signaling. In the proposed approach, the Train Localization On-Board Unit (TLOBU) provides continuous along track positioning and track identification thanks to two separated functions which interact together. These two functions are described in terms of possible requirement metrics, interfaces and algorithms. A particular focus is made on the performances of these algorithms in function of the quality of their nominal inputs. The problematic of integrity monitoring and the sensitivity of the proposed solution with respect to feared events is qualitatively discussed. It is admitted that such approach is more complex to introduce in European Rail Traffic Management System (ERTMS) compared to the virtual balise concept which has been the approach preferred during the past few years.