The Bachmann Company recently introduced a beautiful line of narrow gauge 1/4" scale trains that run on HO track (Peco makes a very nice On30 flex track with scale ties at appropriate spacing). The smoke stacks on the Bachmann locomotives are generally too tall to clear the bridges on the Timber Ridge layout. Consequently, you have a choice of trimming the smoke stacks down a bit or raising the roadbed for the track on the upper loop, thereby raising the bridges. This can be done by glueing strips of HO cork roadbed (shown in brown on the track plan) over the molded roadbed as it approaches the left and right bridges , then sanding the cork at each end to a long, gradual taper, blending it into the molded plastic roadbed. You will also need to add cork roadbed between the bridges on the upper loop but do not sand it.
In addition, the bridges (shown in blue) will need to be widened in order to accomodate the wider car bodies. If you install the optional mountain/tunnel unit, you will need to enlarge the tunnel portals, also to clear the tall smoke stacks and wider car bodies on the On30 locos and rolling stock. The molded portals can be cut away and new portals (available at your local hobby dealer) can be glued to the mountain structure.
The village and industrial center will be restricted to fewer buildings because 1/4 inch scale buildings occupy four times the ground area of HO-scale buildings.