Neutral beams are injected into fusion devices through narrow transition ducts that are permeated by strong magnetic fields. As a result, the beam particles that undergo reionizing collisions with thermal gas molecules within the duct are deflected by the magnetic field onto the duct wall. This phenomena in conjunction with other related effects has been responsible for significant reduction in the amount of beam power reaching the plasma. This paper describes the development of a self-consistent beam transport model that was extensively used in the design of neutral beam injector system for the TDF central cell.