A shift rail interlock system is disclosed for a transmission assembly having a gear unit with a plurality of selectable gear ratios. The interlock system includes three shift rails that are each movable to select between respective gear ranges. Two of the shift rails cooperate to define a plane. The third shift rail is a creeper rail located in a position that is out of the plane defined by the first and second shift rails. The interlock system includes a creeper interlock that is moveable between lock and release positions, which respectively prevent and permit movement of the creeper rail. The interlock system also includes a range rail interlock that is moveable between two different lock and release positions. The range rail interlock is coupled to the creeper interlock such that when one of the range rails or the creeper rail is in the engaged position, the others are in their respective locked out positions.