The hangers most extensively used at present are the =trolley type=, of which the _Coburn_ is the original and the best of the various kinds. The track, a typical section of which is shown in Fig. 121, is made of sheet steel, which is bent or folded into various forms, depending on the particular make. The carrier is contained in the interior of the track. The several features of this type of track and hanger are illustrated in Figs. 122, 123, and 124, which likewise show the method of attachment and the detail construction of the door head, or soffit, necessary to receive these tracks and hangers. This type of sliding-door trolley track is also sometimes lined with wood placed in the steel trough, or track, as shown in Fig. 125. This makes the device absolutely noiseless, although the regular types operate with very little noise. Besides the Coburn trolley and track, there are the _Richard_ and the _McCabe_. Both of these makes possess merit. The trolleys, or carriers, are constructed with both fiber and iron wheels, with ball bearings, running on a wooden or metal track, according to the type. [Illustration: FIG. 122] [Illustration: FIG. 123] [Illustration: FIG. 124] [Illustration: FIG. 125] [Illustration: FIG. 126] This type of hanger and track is made in all sizes, from the smallest, for hanging small bookcase doors, up to the very largest, for warehouse doors. As shown in Fig. 126, trolley hangers are made for doors of special design, such as accordion and parallel doors used to close up large openings. They are also made for elevator doors, freight-car doors, barn doors, and automatic fire-doors, with special construction to suit the various purposes. [Illustration: FIG. 127] In factory and similar buildings, it frequently happens that one portion must be separated from another by brick fire-walls and that the openings in these walls have to be closed with tin-covered doors as required by the National Board of Fire Underwriters, the object being to reduce the fire hazard. Wherever possible, these doors are made sliding and arranged so as to close automatically, being hung on a slanting track with an incline of ¾ inch to the foot, and also counterbalanced with weights so that the door will stand at any point, as shown in Fig. 127. The cord or rope attached to the weights passes over a pulley and is attached to the door with a fusible link, as at _a_, which, in case of fire gives way and allows the door to close automatically. The doors are usually constructed of seasoned white pine or similar non-resinous wood, using three thicknesses of ⅞-inch matched boards, the outside layers to be vertical and inner layers horizontal and thoroughly fastened together with wrought-iron clinch nails, driven in flush and well clinched. The doors are then covered with 14" × 20" IC bright charcoal tin plates of not less than 107 pounds to a box of 112 sheets. All joints are locked ½ inch, without soldering, and nailed under the seams.