nl-2011-07-ponder

A Steel Deck plate girder bridge with steel trestle legs

Things to Ponder When You're Planning Your Model Railroad Layout . .

Bridges – Mechanical and Aesthetic Considerations

by Raymond G. Potter

A Steel Deck plate girder bridge with steel trestle legs

A Steel Deck plate girder bridge with steel trestle legs

As exciting as a model train running around a circle of track might be, eventually the simple thrill runs out, and the train aficionado will want to move on to a more interesting and engaging track plan. One of the first things the novice train engineer will want to add to a layout is a bridge (or even several different bridges).

Bridges can add a sense of eye catching wonder and majesty to almost any layout. Often a single bridge, or a complex series of bridges, can be one of the main features, or even the centerpiece of a layout.

Bridges on train layouts usually span one of two obstacles, either scenic openings such as valleys, depressions, or gorges, frequently containing rivers or roads, or other train tracks. When crossing a scenic obstacle, the main job is to create an attractive bridge structure that blends in with the surrounding scenery and the theme of the layout. When crossing other tracks, there is the additional responsibility to provide adequate clearance for the trains passing below or risk impeding the functionality of the layout. And of course, any bridge you use must be strong enough for the job it will be doing. As with anything, careful planning can avoid a lot of problems.

This stone viaduct bridge is common in the Eastern United States

This stone viaduct bridge is common in the Eastern United States

A wood trestle bridge is more common out west or on small branch and logging lines

A wood trestle bridge is more common out west or on small branch and logging lines

The theme of the layout, as discussed in this column previously, is the time period and geographic location that is being depicted by the scenery, structures, and the trains themselves. Even imaginary railroads will have some reference to a real-world area. Different bridges predominate in different parts of the world. If you are modeling 20th century Europe, it is unlikely that you would find a wooden trestle bridge. Likewise, deck arch truss bridges are scarce in some areas of the United States, while America has a lot of deck plate girder bridges.  The American southwest doesn’t have a lot of stone viaducts, while the eastern US and Europe have many. On the flip side, knowing the type of bridges you really want to see could help determine the location and period for your overall theme. Refer to travelogues, books, and magazines for views of specific geographic areas, or Google search images of the type of bridge you want and see what locations it appears in.

Clearance is an important operational consideration for positioning the bridge. Ultimately a train layout that cannot effectively run trains is not very useful. When we talk about clearance, we are referring to how much space a train needs to safely go through an area. This can be height clearance ie:  you must leave enough space between the bottom of your bridge to clear the tallest piece of rolling stock, or it can be lateral clearance, and you must leave enough space on either side of the track to accommodate your widest piece of rolling stock.

Clearance is especially crucial on curves. A long passenger car traveling through a curve can hang out a good way into the inside of the curve. Likewise, a train pulled by an articulated steam locomotive with long boiler can severely overhang across the outside of the curve. This makes positioning scenery or supports for the bridge adjacent to the lower track being cleared very tricky. One solution is to test any track passing below or across the bridge with the longest or most extreme rolling stock and locos that you intend to run. This method is generally the only way to effectively test clearances for model railroad gauges where there are no firmly set standards. On the other hand, the NMRA has set firm standards that most manufacturers conform to for HO and N scale, as well as two-rail O Scale. In these scales, check gauges are available that you can use to verify all the necessary clearances when you are planning a bridge. At SMARTT , we follow these disciplines as a matter of course in our layout development process to head these problems off before a single section of track is laid. Ignoring any of these considerations could possibly lead to damaged rolling stock, and almost certainly limit functionality of the affected rail lines over a broad range of equipment.

The rusty deck plate girder bridge is ideal for a curved track. There's no worry about swing-out from the train running across it. Below, there is more than adequate clearance for the tallest rolling stock.

The rusty deck plate girder bridge is ideal for a curved track. There’s no worry about swing-out from the train running across it. Below, there is more than adequate clearance for the tallest rolling stock.

Clearance of the track running across the bridge or underneath will also affect the type of bridge you choose. While almost every bridge will work on a straightaway, bridges that have sides or girders raised above the rail level, like a plate girder or a through truss will have to be checked for clearance through the bridge if the raised sections are very close to the track curves. You do not want the sides of your car or the swing-out of your locomotive to hit the inside face of the bridge. The easiest bridge to use on curves is a deck plate girder. You could also use a deck truss with short sections. Trestles can easily be built for curves as can stone viaducts. You may notice that all of these styles keep their structure at or below the level of the top of rail. If clearance to the track below is a consideration, then bridges that have their structures hanging well below the rails of the bridge track are problematic. This could include deck trusses and box trusses.

Clearance of certain bridges that cross tracks can influence by the theme and the era you are modeling. Earlier time periods will have lower clearances for their bridges, as modern railcars can be much taller and longer than those of a half century ago. If you go with a bridge that offers limited clearance, be prepared to limit the variety of rolling stock you can run on it to lower, older, and shorter cars. If you want to run modern rolling stock, plan on a bridge that gives you greater clearance below.

The through truss bridge is coming off a curve at the far right here. The designer has left enough room for the locomotive to straighten out before the loco has to enter the enclosed truss structure. Below the bridge, clearance is tight, so not much of the bridge frame structure hangs very far below rail level of the bridge track.

The through truss bridge is coming off a curve at the far right here. The designer has left enough room for the locomotive to straighten out before the loco has to enter the enclosed truss structure. Below the bridge, clearance is tight, so not much of the bridge frame structure hangs very far below rail level of the bridge track.

A final consideration is the mechanical and structural integrity of the bridge itself. Make sure that any bridge you buy or build can support your heaviest rolling stock and equipment easily or else you may be courting a rail disaster in the making. Elegant and delicate looking bridges may in fact not be tough enough to stand up to heavy locomotives passing over them. This is especially true in the larger scales where motorized equipment can be quite heavy. Good bridges will be made of strong wood, plastic, or metal with firm glue (or weld) joints and no flimsy support structures. If you doubt the integrity of your bridge, either find a way to reliably reinforce it or replace it.

Next up Bridges – Abutments, Piers, and Mix ‘n’ match . . .


from the July, 2011 SMARTT Newsletter

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