When planning and building model railroad layouts, minimum vertical clearances for trains have many implications. At the very least, you'll need to make sure that all of your tunnel portals, signal bridges, structures, and other overhead obstructions are high enough to accommodate the height of the trains used in the model gauge you work with. And if your plans call for an over-under track arrangement, such as a figure-8, helix, or multi-level layout, then the minimum vertical clearance will also impact your grades as trains climb between levels. You'll need to consider both the vertical climb along with the maximum grade when planning your layout.
Obviously, model trains of different scales will have different minimum vertical clearances. These clearances have traditional measurements assigned to them by the modeling society, but real-life history also plays a role in variations to the standard prototype/model ratios.
In addition to the traditional vertical clearances assigned according to the scale of the model, the real-life prototype of the model also has a big impact. Trains have been growing in size since their inception. Early railroad builders could not have imagined that trains would one day stand as tall as the current modern double-stack cars, and they certainly did not build railways to accommodate such sizes. Since the 1980s, railroads have invested millions of dollars in clearance improvements to accommodate increasingly large rail cars.
This new tradition plays a large role if you choose to model modern railways, since you must be conscious of the trend toward bigger, taller, and longer, and build your layouts accordingly. If, however, you model an earlier era or smaller trains, such as narrow gauge or industrial railroads, not only will your trains require lower vertical clearances, but they may actually look better and more realistic with tighter spacing.
Still, oversize loads have always been a part of real-life railroad operations, and creating layouts to accommodate these unique cars and loads can make for very enjoyable modeling projects. If you think you'll be including cars of this type on your layout, plan accordingly.
No matter what your scale or modeling era, a safe practice is to build a test train of your tallest and longest equipment to test the layout as you build. This will allow you check both vertical clearances, and also check parallel tracks and scenery for lateral clearance in curves.
National Model Railroad Association Recommendations
The National Model Railroad Association has standards and recommended practices for just about every aspect of the hobby, including vertical clearances. The table below is based upon their recommendations. For more detailed information, see their Standard S-7.
|1/1 Scale (Prototype)||17 to 23 feet (5.181 to 7.01m)|
|Large Scales*||6 3/8 to 9 17/32 inches (162 to 242mm)|
|O Scale||4 1/4 to 5 3/4 inches (108 to 146mm)|
|S Scale||3 3/16 to 4 5/16 inches (81 to 110mm)|
|HO Scale||2 11/32 to 3 5/32 inches (59 to 80 mm)|
|TT Scale||1 11/16 to 2 5/16 inches (43 to 58mm)|
|N Scale||1 9/32 to 1 23/32 inches (32 to 44mm)|
|Z Scale||1 3/16 to 1 1/4 inches (30 to 32mm)|
*Large scale trains include a variety of scales that all operate on the same gauge of track, commonly called G gauge. The scale of the models vary by manufacturer, hence the greater variation in these minimum requirements.
Remember, these are only recommendations for bare minimums. You may choose greater vertical clearances in some areas for scenic effect or to provide access to the trains (our fingers don't scale down). Also, these figures do not take into account any additional height required for the roadbed and benchwork used to support your tracks.
Another option for those who desire the most accurate scene possible is to refer to engineering track charts for the railroad you model. These are relatively easy to come by online and through railroad museums and historical societies. You may even find drawings for major bridges and tunnels—your most likely modeling candidates. Keep in mind, however, that while model trains themselves are usually close to exact scale, the scale of rails, curves, and even the ride height of models is often compromised, and this can impact the clearances around the trains.