Centerbeam Flatcar | Basic Construction

Introduction

Part of the plan for my BNSF Lakes Subdivision layout includes modelling the Potlatch operation just west of Grand Rapids, MN in the 1980s. Potlatch was a manufacturer of Oriented Strand Board (OSB), a sheet material fabricated from wood chips which are combined with a resin and compressed under heat and pressure. The material is shipped in 4' x 8' sheets and used for home construction. I don't have any photos of carloads coming out of the facility, but it is likely that Burlington Northern used their "opera window" centerbeam flatcars manufactured by Thrall. 

These cars are available commercially as ready-to-run, but I thought I would take the opportunity to build three of them from scratch in N scale. I have never scratch built any freight cars before, so everything here is new to me. At this point, I don't expect to create something that I would have evaluated for the NMRA Achievement Program. They should be adequate to meet the rolling stock requirement for the NMRA Golden Spike Award though, which only requires the display of six scratch built, craftsman, or detailed commercial kits.

Underframe

I started by creating CAD drawings based on diagrams of the 60-foot car on the  BNSF Centerbeam Flatcar page. The floor was cut to size from a sheet of 0.005" styrene from Evergreen Scale Models. The material can be tricky to work with since it is so thin. The underframe is fabricated from pieces of 0.005" styrene strip which are fastened to the bottom of the floor using Plast-I-Weld and a Touch-N-Flow applicator from Flex-I-File as shown in Figure 1. 

Figure 1 - Underframe Assembly

After the basic frame is in place, webs for the side beams are cut to length from styrene strips and fastened to the ends of the cross beams, three scale inches in from the sides of the floor. Solid pieces are built up from styrene strip and added to the bolster ends where the trucks will eventually be mounted to the frame.

One problem with flatcars in N scale is that it is difficult to add any weight to them, so I decided to try using tungsten putty. Tungsten is a strong metal that has a higher density than lead. When ground up and mixed with a putty material, its density is similar to lead. As a putty, it is non-toxic and can be shaped and molded into any shape. I used the Derby Car Supply tungsten putty weight from Hobby Lobby. The putty was rolled into a log and squeezed down the middle of the underframe and between the webs of the bolster beams as shown in Figure 2. There is space between the cross beams for additional weight, but that area will remain clear for now.

Figure 2 - Tungsten Putty

Web plating cut from 0.005" styrene is added down the middle of the underframe and over the bolster beams to cover the tungsten putty and create large box beams. An additional layer of 0.005" styrene was added later over these areas due to issues with warping of the material. Flanges are cut from styrene strip and added to the tops of the cross beams and down the middle of the webs on the side beams before starting work on the bulkheads on the top of the car.

Bulkheads

The backs of the bulkheads are cut to size from 0.005" styrene sheet. Webs for the vertical and horizontal support beams are cut to length from styrene strip and fastened directly to the backs of the bulkheads. Flanges cut from styrene strip are attached down the middle of the webs to complete the bulkheads as shown in Figure 3. Note that the web of the top beam is flush with the edge of the back plate and that the flange creates an L-shaped beam with the flange corners cut at an angle. The webs and flanges on the sides create what looks like an I-beam.

Figure 3 - Bulkhead Assembly

The bulkheads are fastened to the top of the floor at the ends with the support beams facing out as shown in Figure 4. There are a lot of extra details that still need to be added to the bulkheads, including the ladders, cross walk platforms, stirrups, grab irons, and of course the brake wheel/chain on the B end of the car. These will probably end up being commercial parts. I will decide which details, including the brake system on the underframe, will be added later. Right now, I'm just working on the overall structure of the car and trying out different fabrication techniques.

Figure 4 - Bulkhead Installation

Center Beam

Like the underframe and bulkheads, the center beam starts out as a sheet of 0.005" styrene sheet. The "opera windows" are a unique feature of this car. They are cut out from the styrene sheet using the CAD drawing as a template as shown in Figure 5. Webs for the vertical support beams are cut from angled strips of styrene and fastened to the sheet with the wider end at the bottom. The beams are angled to tilt the top of the load toward the center of the car. Flanges are cut to length from styrene strip and attached down the center of the webs.

Figure 5 - Opera Windows

The center beam is attached down the center of the floor and to the centers of the bulkhead plates as show in in Figure 6. The top of the center beam is flush with the tops of the bulkhead plates. A flange is cut to length from 0.005" styrene strip and fastened to the top of the center beam. At this point, the floor straightened out from end to end but the car could still be twisted, and the top of the center beam could be flexed sideways.

Figure 6 - Center Beam

Top Beam

To stabilize the structure of the car, the prototype has a top beam across the top of the center beam, creating what is essentially a very large I-beam. The undersides of the top beam are cut from styrene strip and fastened at a shallow angle, creating a trough along the top of the center beam. An additional web and flange are cut from styrene strip and added down the center. Narrow strips are added along the outside edge of the top beam and short strips are added across the top to hold everything in place as shown in Figure 7. At this point, the car becomes relatively rigid.

Figure 7 - Top Beam

To complete the interior of the car, thicker strips of styrene are added to create support beams along the floor and angled to match the angle of the vertical supports. The support beams on the floor and the vertical support beams create a 90-degree angle that is tilted backward. Webs and flanges for the horizontal support beams are cut from styrene strips and attached between the vertical supports on the ends of the car.

Conclusion

I drilled holes in the solid areas in the bolsters underneath the car and added a set of N scale Barber Roller Bearing Trucks (part number 003 10 040) from Micro Trains Line as shown in Figure 8. These have truck-mounted couplers and 33" wheels, where the prototype has 36" Griffin wheels and ASF Super Service RC trucks. I was going to replace the wheels anyway, but going to bigger wheels will lift the coupler by about 0.010" so I'm either going to have to go to body-mounted couplers or find truck-mounted couplers with the bigger wheels. I ordered some BN centerbeam flatcar decals from Micro Scale so I should be ready to start experimenting with painting and decaling the car.

Figure 8 - Final Result