Direct braking system is a key part of train security. This system is complex and nonlinear because of coupling multi-physic energies (pneumatic, electric, mechanical translation and rotation) and working in hybrid structure involving continuous and discrete modes because of presence of relays and diodes. The objective of the present paper is to use one representation: a hybrid bond graph as a multidisciplinary and integrated tool for dynamic modelling of all modes but also structural diagnosability analysis to improve the preventive maintenance, to insure the reliability and availability of the train for its health management.