Abstract: Apparatus and methods are provided for delivering a lead over a rail into a target body lumen, cavity, or other vessel, e.g., a coronary vein or a right ventricle within a patient's heart. For example, a distal end of an elongate guidewire or other rail may be introduced into the coronary venous system via the coronary sinus, advanced through the coronary venous system to a location beyond a target vessel, and secured at the location beyond the target vessel. A catheter or other elongate tubular member is advanced over the rail and manipulated to position an outlet of the tubular member adjacent to or otherwise aligned relative to the target vessel. A distal end of a lead is delivered through the tubular member and out the outlet into the target vessel. The catheter and rail are then removed, leaving the lead within the target vessel.