I built a similar car in N-scale and it proved to be very useful for "debugging" turnouts. I am getting ready to work on the track of my first S-scale layout, so I decided to take the time to build one of these in S-scale. Using a "car" like this helps me figure out how the wheels behave as they go through a turnout or crossing.

To be able to see how the wheels respond to the track, a see-through material is needed for this special "flat car". I decided to use some plexiglass that I had available. Using the table saw, I cut it to the typical dimensions of a 40-foot freight car (1-3/4 x 7-3/4 inches).

The other parts needed for the car are two trucks and a pair of couplers. The couplers are optional, but I have found that additional track problems can be discovered by coupling this car to another car and then pushing or pulling this inspection car through a turnout using the other car. That is why I decided to add the couplers. I am using a pair of Kadee #808 S-scale couplers for this car. The trucks can be any representative truck and wheelset used on the layout. I decided to use the S-Helper Service's "PRR D2-F8 50 Ton" trucks with code 110 wheels (part #00047). These are heavy trucks that roll beautifully.

The trucks need to be mounted to the piece of plexiglass. Normal cars use a bolters, but for this inspection car we don't need one. I found two screws and matching nuts from the parts box and installed them onto the truck. The screw needs to be long enough to allow the plexiglass to clear the wheels and then allow an additional nut to be placed on top of that. The two nuts are tightened against each other, which keeps them from moving up and down the screw as the truck turns. Leave enough play to allow the truck to turn on the screw.

Next, a hole needs to be drilled at both ends of the "car" to take the screws. I marked a drill point in the center and 5 scale feet away from the end. I first drilled a very small pilot hole before drilling with the final drill bit. I made the hole just wide enough to clear the screw. If it is too wide, then the car will wobble too much. Using a knife I cleaned up the debris from the drilling.

The next photo shows a side profile of the installed truck. A nut at the top secures the truck to the plexiglass. And, voilą, we have an inspection car.

I then assembled the Kadee couplers and glued them to the plexiglass car using 5-minute epoxy. I don't have the right size tap otherwise I probably would have used the couplers' screws to install them. The glue works fine too, though. You can manipulate the trucks' screws to get the body to be at the right height so that the couplers will match other cars on the layout. Update: I found out that the screws are actually self-tapping. The glue didn't hold the coupler boxes for but two weeks, so I drilled a hole and put the screws in place. It worked great. (the next photo shows the coupler glued)

The next photo shows the completed car. Now it is time to start laying some track.

Another good use for this car is to verify that track is level. Make sure that the wheels are even or otherwise the car will always indicate not level track.

This is a rather simple project to build, if you have the parts. For my old N-scale car I had to add weights to the center of the car to have it track reasonably well. I am using a remove metal bar for weight. That way it doesn't interfere with the see-through feature of the car, and the weight can be used for other purposes when needed. Gluing something more permanently may not be a good idea, because anything placed on the car may obstruct the view of the trucks' behavior over track.