The Flatcars not Flat

I introduced my project to model the shipment of the Dominion of Canada from Exporail in Montreal to the National Railway Museum in York England here.  Obviously, a key part of this project, is the heavy load TTX flatcar that the locomotive was loaded on for shipment from Montreal to Halifax and the ship to take her back across the Atlantic Ocean.  No Ready-to-Run model from a major manufacturer exists of this type of car, and I couldn’t find any evidence that any resin or small run manufacturer has done the car, so I took it upon myself to design it as a 3D printed car.  For the most part, after some struggles with how to design the span bolsters to support the trucks, getting the dimensions right so the car would ride at the right height, and the couplers would be in the correct heights, I thought I had gotten close.  The one big issue was how to get weight on the car.

To accommodate weight, I designed the centre section of the car to have a cavity between the frames where a combination of 1/4 ounce and 1/2 ounce lead weights could be hidden.

Shapeways Render of QTTX131344 showing location of cavity for weight.

The second opportunity for weight is to be hidden inside the wire frame A4 locomotive that will be riding on top of the car. The one concern with weight here is that it will make the car top-heavy and tip prone as the weight will be well off the rails.

When I test ran it on a friends layout, it had some tracking issues as all the weight is in the centre of the car, which is prototypical, but in 1/87th scale, doesn’t necessarily work so well.  There is some limited room in the span bolsters to put some moldable lead weight into them without impeding the space needed for the wheels to move, which I will do whenever i get my hands on some moldable lead weight!

I thought I had sorted the weight issue, but then I took the car out last week to start weathering and rusting the deck, and I noticed a major problem, the little sag I had notice some months ago when I last was working on the project, had become a major sag that would no longer be acceptable.  I suspect, that this is a result of a design flaw in my 3D print, on a few different grounds.  I had to cut out some of the supports in the original print so the trucks had enough room to move, in doing so, I likely hurt the strength of the material.  I’ve found that Shapeways FUD material can be drilled for putting in details, but doesn’t seem to like being cut much in terms of the material retaining strength.  A second problem, is that the deck of the flatcar is only just at the minimum thickness to print properly, and be close to the scale thickness of the deck.  The third problem, is I suspect I ignored a lesson of those who originally designed flatcars for the railway, that they need a strong centre beam below the deck to prevent sagging and maintain structural rigidity.

3D Print of QTTX Flatcar 131344.  The sag in the middle isn’t as obvious in a broad shot, but it is there.

When the deck is set upside down the bowing is much more noticeable.

The tip up is very obvious in a closeup of the span bolster area.

The good thing is, I haven’t put the car for sale through my Shapeways Store, so I haven’t sold anyone a car with a flawed design.  As well, once i determine the right fix for the deck, ordering a new one and painting/finishing it is easy enough to do.  The bad news is, I’m not 100% certain what the right approach to a fix is.  There are a couple of different options.  My recollection is that the car was perfectly flat when it was received, which means its warped over time from a combination of washing with warm water and soap to clean the support material, painting, the natural tendencies of the material, and potentially exposure to environmental factors (i.e. the heat in the apartment).  Not knowing which is the root cause, or the percent to ascribe to each potential issue, is going to make a fix tough.

The one universal change I am going to make is to thicken the deck between the frames, to make it as thick as the thinnest part of the frame.  I’m trying to strike the balance between cost and benefit.

Beyond this, to try to tackle the major issue of the bowing, the two most straightforward options I can see, are to re-print the car as it was and immediately inset a brace, or to re-print with increased internal supports.

For the first option, instead of the weights I used, i envision take a piece of flat steel bar, cut to fit precisely between the two bulkheads under the car. This would provide both weight, and hopefully, structural rigidity.

For the second option, as rendered below, there would still be channels on either side of a new centre beam to place weight

Potential re-design to add a centre beam in the weight cavity.

I’m no engineer, but I have a basic understanding of structural design.  I think the above will make a stronger car in that having the integral bar will help eliminate the bowing (or hopefully reduce it).  Anyone with more experiene in structural design’s thoughts would be appreciated.  Once I settle on something and eventually order it, I’ll report back, but that will be some time down the road.