Thursday, August 19, 2021

Full-width diaphragms, Part 3

 This is the third post in this series about the full-width diaphragms that were adopted by a number of railroads including Southern Pacific, for new cars in the early days of streamlined lightweight passenger trains. In the first post, I described and illustrated the prototype, with several photos to illustrate the features of these interesting appliances. You can see that post here:

In the second post, I made a few observations about commercial products that can represent the full-width prototype diaphragm, either in flexible, working form or in a rigid plastic form that does capture the appearance, though not all aspects of the operation. Here is a link to that second post: .

Now I want to turn to face plates for these diaphragms, particularly the distinctive shape of many of the ones used by SP. These have sometimes been called “opera window” plates because of the peekaboo openings at the top (possibly to save weight). Below is a repeat from the first post, a clear photo of such a face plate on a 1947-built SP car.

One way to make model versions of these plates is simply to cut them from styrene, after making a pattern. I made patterns by taking a photo like the one above, increasing the contrast using Photoshop, and then sizing the height of the plate in the photo to 9.5 feet in HO scale (about 1-5/16 inch). Here’s the relevant part of the photo above.

Next I printed out this image on ordinary paper, cut it out with a hobby knife, and then transferred the outline with a pencil to 0.010-inch styrene sheet. Shown below are a paper pattern, cut from the printed-out photo above, and a “first-cut” styrene piece, obviously needing refinement, but I want to show it at this stage to show you don’t have to get it perfect at the start. 

The starting blank at right has too high a curvature at the top, and the upper holes are too small and poorly shaped. But with the thin styrene sheet, these defects are quickly corrected with small round and flat files.

Shown below is the same “blank” shown above, but refined in shape and primed with a medium gray color, prior to preparing for use on a model car. This still needs rust tones added.

This will be attached with canopy glue to a full-width diaphragm, or to a residual “narrow” remnant diaphragm. By the latter, I mean what remained after the removal of the full-width part of the diaphragm. The photo below shows such a diaphragm and face plate, on a single-unit diner, SP 10200 in 1956 (Doug Richter photo). You can click on the image to enlarge it if you wish.

I have made a number of these “opera window” face plates, for use on full-width or remnant diaphragms for appropriate cars. I will go into their application in a future post. 

Tony Thompson


  1. I’ve found FWD’s interesting, given that my somewhat dormant passenger car interest is c.1940 PRR.

    The thing that strikes me is that they are a 3 piece component; they have essentially got a conventional diaphragm in the center (with the oversized faceplate), and then two hinged “wings” on the outside. Presumably this means that on tight curves on a real car, the “conventional” diaphragm and the “wing” on the inside of the curve would remain firmly in contact, but the “wing” on the outside of the curve would probably remain essentially parallel with the end of the car.
    I don’t have any bright ideas of how to model that geometry (getting the flexibility and low spring force required would be tricky), but it strikes me that if the faceplate on a diaphragm like the Coach Yard ones was hinged, it would be easier to keep them in contact.

    1. Actually, Ed, I think that on the prototype, the outer diaphragm parts remained in contact at ALL times, and in fact you can see clips at the other edge that made the connection. Our model curves are immensely sharper than the prototype, so it can be hard to image.

      In fact, the Coach Yard and BLI full-width parts do work for "broad" model curves. No need for a hinge.
      Tony Thompson

    2. I’m curious as to why the outer “wings” were hinged? It seems like they were pretty consistently in 3 parts; presumably the clips were for on the main line and the hinges were perhaps for being shunted on tight curves when the outer part would have to be overextended?

    3. The center part remained in contact with the adjoining car to enclose the passengers' passage. It could flex within limits. The outer part would have a greater range of motion, while still remaining in contact with the adjoining car. That way, the outer edge could be closed up, or extended, relative to the center. Or at least that's how I understand it.
      Tony Thompson