The term tier trestle is fairly recent, given by historians to the bridges built along the route of the Burma Thailand Railway (the River Kwai). See, for example, this Australian government report.

The term could, of course, refer to what are called the levels or stories on a standard trestle. (The Oxford Dictionary defines tier as one of several units of a structure placed one above the other.)

There are key differences with the tier trestle and the engineering standard. The river bridges built by the Imperial Japanese Army engineers using prisoner of war and indigenous slave labour did follow engineering standards and were solid enough to survive repeated attacks by British and American bombers.

On the other hand, the bridges over the hundreds of ravines were not-so-solid, built hastily and by engineering units that were not as experienced or competent as those building the actual river bridges.

Recent scholarship seems to indicate that the long-term Japanese plans called for these bridges to be filled and covered with earth. a method described in the Merriman Wiggin American Civil Engineer's Handbook, an indication that the similar methods were used in North America.

Steam era modelers may want to use the tier trestle bridge or some variant for what the Handbook calls "construction trestles" on temporary lines, or on narrow gauge railways. The handbook recommended removing the bracing during the fill process when it is reached by the fill to prevent the pull in the bracing under the load of the fill from distorting the posts and even breaking them.

In muddy conditions, common in the rain forest, it warned that "if piles were driven into the mud, sudden movements of the mud and newly made fill frequently not only break the braces but snap off the piles and demolish the entire structure. Such trestles should be built of piles of large diameter, driven to hardpan and heavily sway braced and the sway bracing removed when reached by fill."

Due to war time pressures, in many cases, this step was never completed and so the tier trestle bridges were used until the line was abandoned, often requiring frequent repairs.

Characteristics of a "tier trestle."

• Longer caps and sills, and a longer horizontal bent brace at one level reaching out beyond the normal exterior bent posts.

• Additional vertical bent posts along the outer edge with horizontal girts that reinforce the bridge between the caps, sills and horizontal bent brace.

• Additional bracing from raw logs that are attached to the ends of the caps, sills and bent brace.

• Additional diagonal bracing between the platform and the cap. These braces were found on all the bridges built on the Burma Thailand Railway, not just the ones over ravines. In some cases, there sometimes occasional longer bridge ties visible in old photographs, indicating that there may have been bracing on there as well.

The Merriman Wiggin handbook makes no mention of this type of construction, so it may be that the Japanese engineers improvised at first and then adopted the method along the entire railway.

Materials 

The Japanese engineers used local hardwood, mostly teak, to build all the bridges. In my preliminary research experienced bridge modelers told me that dowels that are true to N scale are not available.

Finished logs

• bamboo skewers available in any supermarket

• round toothpicks

• parts of bamboo place mats. (I was able get them very cheap from a Japanese shop in Toronto. They had been used for window display and so came in various shades even on the same mat due to fading in the sun.

Squared wood.

• Kit bashed from the Hunterline N Scale 81 foot trestle bridge.

Support logs

• At first I couldn't figure out how to do the logs that support the bridge. The solution came from another technique, while I was using the blender to turn old leaves to dust for ground cover, as a number of modelers have recommended. Those instructions say discard the stems. But as I was cutting the stems off, I realized they were prefect for the tree trunks that were used to support the bridge.

Track

• Micro Engineering bridge flex track

• Centre planks (HO size) 2x2 lumber

• Tie supports (HO size) 1x3 lumber

  

Staining the wood The tropical hardwoods, like teak, used in the building of the bridge are usually insect and rot resistant without use of preservatives such as creosote. And creosote was not available during the war. So I stained one third of the bamboo skewers, toothpicks and Hunterline stringers and sway braces with teak stain. Another third was stained with teak stain mixed with neutral for a lighter shade. The final third was left natural. The bamboo place mats were left natural.

Building the bents Construction methods on the ravine bridges were hurried and often slipshod. One called "The Pack of Cards Bridge," built by conscripted Burmese forced labour under Japanese supervision collapsed three times. So the building of the model also reflects this. The caps, sills, exterior bent posts, abutment posts and retaining walls were made from bamboo skewers. So were the extra horizontal exterior girts. Bent posts were made from round toothpicks. The horizontal bent braces from the bamboo mats. Other bent braces, girts and stringers were from the Hunterline kit. I followed the instructions from Hunterline and from various articles in hobby magazines, building the bents first.

The track I found that Micro Engineering instructions can be awkward and it is best to add the guard track first, using CA adhesive and small clamps to make sure it is well stuck. I used only one guard track, rather the regular two. One guard rail or shorter ones are common in old photographs, probably due to shortage of materiel. Planks were glued in the centre of the bridge. The railway was the only route through the jungle and many people used it as a roadway, which meant there had to be places to walk between the ties. You still see the same kind of planks on railways in parts of Thailand today. The stringers were glued to the bottom of the track. Completing the bridge The standard bridge was built first by adding the girts and sway braces. I then added the additional girts

. Weathering To weather this bridge, I had to keep in mind that it will be in a model rain forest. The track was painted a grayish color and then given a teak wash as I describe in the Apalon bridge post. The first step was to use a variety of green chalks brushed onto the bridge. The second step was to use black and gray chalks along the track and on the bents, since this was a heavily traveled steam railway. I had tested the standard weathering mix of heavily diluted shoe dye and alcohol on stir sticks but the result was far too dark for my purposes, based on some of the bridges I saw on my trips to Thailand and in the first colour photographs from the 1950s (WWII photographs are in black and white). Then I remembered that this was rain forest. So I tried a new, two-fisted, approach, black weathering spray, followed immediately by a heavy spray of wet water. The result was a soaked newspaper and a very light grey with occasional patches of black. The two sprays also took away most of the chalks, so after all was dry, I added more green, brown and black chalks.

Support tree trunks The leaf stems were soaked in dilute matte medium as a preservative. There were two levels of support logs, smaller ones attached to the lower tier and the second tree trunk logs to the upper tier. The stem/logs look good in the photograph but are quite delicate. Once all the bridges are ready and the track laid and glued, the stem/logs will be cut to a proper length and anchored.

The spider webs. A glue called Liquisilk was distributed as free samples at the Toronto Christmas Train Show so I tried it out when adding the tie supports where I wanted to test the company's claim that the glue could be strong in small amounts. It wasn't until I took macro photographs of the bridge that I noticed that there was excess glue, that it did look like spider webs, especially when fragments of chalk adhered to the dried glue. Liguisilk

Finishing At this point, before final installation on the layout, I sprayed the bridge with Krylon matte, then cleaned the track and temporarily installed it on the layout and successfully ran tests with my trains.

Coming up in the next few months A river bridge A viaduct Smaller river and ravine bridges

I have completed the basic layout for the Wampo, Nieke and Sonkrai railway. Before I glue down the track, the bridges have to be built. The first bridge is the simplest, a two-span deck plate bridge on three concrete piers.

The prototype The prototype is the Apalon Bridge, about 25 kilometres inside Burma, beyond Three Pagoda Pass, at the 335 kilometre mark from the railhead in Thailand. John Stewart described it in his book To the River Kwai Two Journeys 1943, 1979, when he visited the bridge on his return journey. At the time it had been abandoned for 35 years, and at that time, appeared to be intact, but appeared to be freshly painted a dirty shade of red, like coagulated blood. From close up, it is revealed to be nothing but deep rust which Stewart says contrasted sharply with the pervasive greenness of the jungle. A photograph shows thick jungle right up to the edge of the pylons on either bank.

Alternate world This project is more of an alternative world, the term taken from speculative fiction than the pure model railroading freelance.  In this alternative world, the railway was not abandoned; instead it becomes a mainline route from China and Southeast Asia, as well as traffic between Thailand and Burma as well as the local runs. However, in the post-war world, there is a minimal budget, and so far, in the period 1946-1947, the maintenance on the hastily built railway is concentrated near the railheads and high traffic areas in Thailand and Burma. The border region that I am modeling is on the list but at the bottom. So the bridge can be described as neglected, and I have weathered it, as it would have been in either world in 1946 or 1947.

A note on construction In the construction of the Burma Thailand Railway, the wooden bridges, ties (sleepers) and telegraph poles were made from local insect resistant tropical hardwoods, mainly teak. At least during the period of the Second World War, creosote was not available and not used. That means the traditional methods of staining or painting both the wooden trestle and the ties do not apply on this railway. Teak and other hardwoods were used, usually untreated, for many years after the Second World War across Southeast Asia. Later various forms of anti-insect treatments were used. Today it is more common to use metal and/or concrete for bridges and poles.

The model The original model is made from two Kato N Scale deck plate bridges with Kato pylons. There is a close resemblance to the original Apalon bridge

The pylons I came upon a method of creating neglected or decaying concrete purely by accident. I was testing Krylon All Purpose White Primer #41315 on some scrap styrene. The result was a powdery cracked white, not all suitable a primer, but perfect for crumbling or neglected concrete. First I sprayed the three pylons with Krylon primer. Once it was dry, I applied a wash of Polyscale Concrete, allowed it to dry and then applied two more washes. The level of the Kwai Noi varies from day to day and sometimes from hour to hour. Flooding is frequent during the rainy season. So how to create flood/mud stains on the pylons? So I tried an experiment, I created a bath of artists acrylics (raw umber and raw sienna), and mixed it so it actually had a consistency of mud. I left the three pylons outside in the sun, which reduced the bath and left a stain, then transferred the pylons to my work bench, where the remainder of the paint bath evaporated over three days, leaving an authentic looking stain. After the mud stains were dry, I applied artists pastel chalks, first some raw umber followed, in the tropical environment with a bright Phthalo Green, an Olive Green and then a mixture of the two. The final chalks were Black, Mouse Grey and a mixture of both. The final step was a Krylon matte spray to fix the chalks and remove any sheen.

The track I have already run experiments with spare Kato Unitrack and Atlas Snaptrack. Both have ties that are too dark to match tropical hardwoods. As is widely recommended, I coated the rails with oil before each painting step. What worked best was Krylon Satin Almond spray #42327, which creates a dull grey-brown finish. The second step was also an experiment. I had successfully tested Home Hardware Teak wood stain on bass and balsa wood prior to building the trestle bridges. (More on that in future posts) 

So I brushed the ties with the teak wood stain"and that worked, bringing out the details of the ties and adding a teak-brown tone to the grey from the spray.

However, this technique works best on track without a built-in roadbed, since the stain tends to bleed into roadbed. (I am working on a couple of other techniques with the Unitrack) I then painted individual ties with a variety of washes from Polyscale D&RGW Building Brown, Depot Buff and Mud, adding a smidgen of Box Car Red now and again. The guardrail and the sides of the rails were painted with Polyscale Rust. I added a wash of rust on the central walkway. It was chalk pastels that made all the difference. First was for rust, Caput Mortuum Red, Indian Red, Permanent Red Deep and Raw Umber (and mixtures of those shades). There were several different shades of Raw Umber in Curry’s Artists Supplies in Toronto, so I used those to add a general aged appearance. As with the pylons, I used my selection of green to add some hint of the jungle, followed by greys and blacks. The spans Kato calls the colour of the deck plate grey. But it was actually a grey green that was perfect for my needs since it closely resembled camouflage paints, likely the only paint available in the region at the time anyway. I used a small sculpturer's pick to distress parts of the bridge, weakening some of the side rails and poking some small holes, which could have come either from allied strafing or just general wear and tear. Again I started with wash of Polyscale Rust, followed by a mixture of Rust and Building Brown, but largely left well enough alone. The main step was a heavy application of pastel chalks, several of mixtures of a rusty orange, followed again by greens and finally by blacks.

Finishing All the elements were sprayed with Krylon matte finish, to seal the chalks and to remove any remaining plastic shine. I gave the Kato unijoiners a thin wash of concrete, and added a black gantry support in the middle, that I may use for a telegraph pole or just leave as is. 

Next step The next step is the first trestle bridge.