Tuesday, September 14, 2021

SPMW 7021A (Part 2) Wrecker - Rail & Tie Car

The previous post on the SPMW 7021A (Part 1) covered my first go at building the bulkheads and rack.  However, after looking at the prototype references closer, the bulkheads were too short.  It would have been easier to cut them down, but not so easy structurally to raise them.


I took some time to draw up the car in my CAD software and get the proportions worked out. Much of the internal structure is guesstimates and logical design based on what is visible in the photos.  I have a couple other photos from Bob's Photos, which I don't have permission to publish here on my blog.

SPMW 7021A stripped of original bulkheads sitting on top of 1:87.1 scale rack plans.

I printed out my CAD drawings and then double checked them against the model that the printer scaled the drawing correctly to match the SPMW 7021A physical model.

Ok, everything checks, time to start!

So, after a break of about 6+ months and now armed with a good set of drawings... I'm looking forward to diving in and finishing this crazy looking project!

Fabricating the Rack

The first step in fabricating the rack was deciding how I was going to model the 6" main span beams.  I have some 0.060" C-channel styrene shape, but I need more like 0.069" tall beams per the scaling off the photos.

I decide to use the 0.060" C-channel stock and glue a 0.060" x 0.010" strip along the bottom, creating a bit of a flange to the inside of the C-channel for the cross pieces to attach to.

I start by gluing the 0.060" I-beams to the 0.060" C-Channel/L-girders.

The end beams are also formed from 0.060" C-channel stock.  I'm using a section of plywood with square marks and basic dimensions transferred over from the drawing so that I don't get Tamiya glue on the paper print.  Later on I get more adventurous about doing this!

Cross frames are glued in place.

The cross frames are made from 0.06" x 0.04" strip styrene, this will allow them to match height with the main frame beams.  Once they are tacked in place at the marked points, I add an additional 0.06" x 0.01" styrene strip to the bottom, inboard of the flange, forming a larger gusset plate.

All cross frames in place with gussets.

Now it's time to make the diagonals.  I would have liked to use 0.04" x 0.01" strip, but I only had 0.04" x 0.02" on hand... oh well.  

I cut and fitted the diagonal bracing

At this point, it is critical to ensure the rack is square before installing the diagonal pieces!  I did all the braces in one direction on the top, then flipped the rack over and did the other direction on the bottom.

Bottom of the "flat" rack frame

Top of completed "flat" rack frame

The process of cutting and fitting all the diagonal pieces is complete.

Fabricating the Bulkheads

Marks for collision posts on bulkheads.

After reviewing the previous material on how I built the first set of bulkheads 6 months ago, I should have used the same spacer method on these when attaching the collision posts to the bulkhead sheets.

Mismatch in height of bulkheads.

I marked the A-end bulkhead on the interior side with pencil to help me align and set the height of the rack frame.

Pair of styrene bulkheads completed.

The top of the bulkheads are finished off on the exterior with a strip of 0.060" x 0.010" styrene strip, which actually helps the 0.010" bulkhead sheet keep from rolling up during handling.

Joining the Rack and Bulkheads

Rack frame with offset end sub-beams.

The prototype photos show that on both ends there's a I-beam installed inboard from the bulkheads, under the frame.  I'm not sure why these are mounted below the rest of the rack, but they certainly are.  I fabricated them out of a couple of 0.060" I-beam stock, cut to the width of the rack.  The lower corners are cut off at a chamfer, so I was sure to do that before installing.  I added a short piece of 0.04" x 0.02" strip between the frame beams and the I-beam sub-beams to account for the thickness of the diagonal bracing.

It was also at about this point that I realized that I would need to cut off the end C-channels from the rack frame, as I had accidentally made the bulkheads too thick, using the 0.06" collision posts, instead of the planned 0.04" strips.

Rack prepared to be installed on the bulkheads.

I slightly miscalculated and used 0.040" x 0.060" styrene strips on the bulkhead collision posts, which pushed them in an additional 0.020" on each end from the CAD drawing, which was expecting only 0.040" x 0.040" strips.  I had hoped to use the C-channels to create a larger bonding surface to the bulkheads... but I guess that's not going to happen!

Rack frame installed.

At this point I start gluing the rack to the bulkheads.  I was double checking the space from the deck to the bottom of the rack main span beam at 0.305" with my calipers before the Tamiya glue fully hardened.

Checking the rack for square and level.

I may still add another 0.060" x 0.040" strip across the ends at the bulkhead to reinforce the inside of the joint before I paint the rack and install the running boards.

Fabricating & Installing the Gussets

I fabricated the two large gussets on the A-end of the rack from 0.010" styrene sheet using the CAD drawing to help get the size right.

A-end gusset plates are cut and installed from 0.010" sheet.

The anti-shear gussets are useful to have, oddly they're only installed on the A-end of the rack.  Funny enough, it actually does make the joint much stronger, forcing the 90 degree angle between the rack and the bulkhead to be fixed.  However the middle of the rack is still quite flexible and able to sag.

Second gusset installed.

Around this time I also marked the centerlines for the various grab irons which will need to be drilled.

Close-up of A-end bulkhead

Grabiron hole marks are also put on both bulkheads, which I'll need to drill out soon.

B-end bulkhead with grabiron marks.

The B-end will also need to have the extended brake staff and rigging installed to the left of the coupler.  I'm not sure how high the brake wheel was above the upper running boards on this car.

Installing Vertical Rail Rack

I predrilled the top of the rail posts with a 0.015" drill bit about 0.04" below the top end.  This will be for the cable "hand hold" between the posts.

The outer edge of the posts are marked on the horizontal frames

I mark the horizontal frames with my mechanical pencil 0.315" inboard from the edge of the rack.  This will be the mark for the outer edge of the vertical rail posts.

The vertical posts are glued in place with Tamiya glue.

The posts are attached with butt-joint connections to the horizontal frames using Tamiya liquid glue.  The glue welds the plastic together, creating a fairly strong joint.  Later on I'll put some splice plates or additional supports in there to beef it up a bit.  The prototype photos show at least some angle-iron or other bits which should support and form a partially open box for the ties.

Using a 1/4"x 1/8" strip of styrene as an alignment guide.

One of the challenges of actually gluing these posts on free-hand is getting them properly aligned. To help with this, I glued the end posts first.  Then I'm double checking the alignment with a straight edge (heavy styrene strip) to see that the posts are still in a good line.

The rack is still rather flexible, so putting the heavy styrene strip inside for support.

The model rack's coming together nicely, but it is still rather flexible in the vertical axis across the span.  I'll be putting in more spacing structure higher up between the posts, but for now I want to put some support in under the middle of the rack to keep it from sagging.

In Closing

The rack is mostly installed now with the vertical posts.

This is where I'm going to wrap it up for now.  Upcoming steps will include fabricating the supports for the running boards, putting in spreader pieces between the vertical rails.  I'll be using 0.009" PB wire for the top cable between the posts.

I'll then mask the main flatcar and paint the bulkheads and rack before installing pre-stained basswood running boards around the top of the rack.

Jason Hill


  1. Hi Jason,
    I'm following your build of the rack car. It's an amazing amount of crossbraces to do with only .060" channels. How about using brass shapes? Would probably make the rack more rigid. Also if you plan to put a load of rails in the rack. Just my 2 cents.
    Keep up the great work. Although I'm a Santa Fe modeler, with a layout based on the Surf Line (LA to San Diego), I plan on building an SP work train based on your great blog posts.
    Alain Kap, MMR
    Saarburg, Germany

    1. Yeah, I thought about brass... but I have the styrene here in stock, and I'm not so sure about getting the brass stock in the US right now. I've done some brass work like that, but the previous model was styrene and has held up ok. It's just tricky to do the assembly work before it's all done. It should stiffen up as I get the rest of the upper works done.

      Yes, I'm planning to do the load for the car, that will probably be in Part 3 or 4.

    2. Thanks. I'll keep watching your posts.


Please identify yourself at the end of your message. Please keep comments relevant to the post or questions to me directly.
All comments are moderated and must be approved, so give me a bit of time to approve them.
No random solicitation in comments. Spamming and phishing comments will be deleted or not allowed to post.