In the European Train Control System level 3, using on-board control-command systems for the train integrity monitoring and length determination functionalities transfer the train operation safety responsibility from the infrastructure managers to the railway operators. To implement these safety-critical functions, quantitative and qualitative safety requirements shall be specified as stipulated in the railway safety European standards and regulations. These functions must fulfil safety integrity level SIL 4 requirements at the system level, independently from trackside infrastructure. The contribution of this paper consists in proposing a methodology to determine the safety targets of both functions in terms of Tolerable Hazard Rates (THR). The obtained THRs are then apportioned among the safety-related functions to specify the quantitative safety requirements for every functional part.