Test mock-up for true scale coupling distance on Branchline Trains® passenger cars. Details such as steps are yet to be applied so we can see what's behind them.

Many have asked the question:

How do I uncouple passenger cars equipped with Sergent Engineering couplers? There is no way to get the uncoupling wand to the couplers because the diaphrams will be in the way!

This page addresses that question specifically for Branchline Trains® absolutely exquisite passenger cars, but the method can be applied to other manufacturer's cars as well. This method will require some modification to underframe of the car, but it's nothing terribly difficult. You will be rewarded with car spacing just like the prototype and prototypical operation to boot! I think you'll agree that the results are well worth the effort. Read on...

This is what we start with...

The photo to the right shows our starting point. The models shown are equipped with the traditional knuckle couplers supplied with the kit. Note the huge gap between the car diaphrams. It's over a quarter of an inch wide! You can see that one would have no problem getting a Sergent Engineering uncoupling wand to couplers that might be retrofitted to these cars. So... if you are in a hurry, you can simply drop Sergent Engineering EC87 couplers into the pockets on these cars and be done. Where's the fun in that? You want prototypical distance between the cars and you want the diaphrams to touch just like on the prototype.

Why is the gap so wide? I have to believe these cars purposely deviate from the prototype in this area for two reasons. Both of them have to do with the customer's (likely correctly in many cases) perceived desires and neither has much to do with the type of coupler that might be installed.

  1. First of all, lets say you have a train made up and for one reason or another you decide to bring in the big 0-5-0 and yank a car from the middle of the train. It happens. The gap allows couplers to slip out of engagement as you lift the car from the middle of the train. Look at the first photo on this page and think about what would happen if you attempt the same move without the big ugly gap. There isn't enough room between the bottom of diaphram and the top of the coupler to allow them to slip apart. The cars won't let go and you'll end up putting the whole she-bang on the ground!

  2. The second reason for the gap has already been answered in the form a question asked by many Sergent Engineering customers. How do you uncouple the train? With the big ugly gap between the cars, you just insert your handy dandy uncoupling wand, skewer stick, or whatever into the gap to release the couplers. If the gap isn't there, you have a problem. If you use traditional knuckle couplers with trip pins, you can move the train to some location where you had enough forethought to install an uncoupling magnet -- but that's pretty inconvienent.

So the gap is there, and it's there for some good reasons. Our goal here is remove the gap, but our solution can't ignore the reasoning that lead to the gap in the first place.

The big ugly gap is not prototypical. Is the gap there because the couplers stick out too far, or because the diaphrams are too skinny, or both? It's not because the diaphrams included with the kit are too skinny. In fact, comparision of those diaphrams with prototype drawings shows that they are very accurate. Branchline Trains® deserves a lot of credit for realizing the need to provide diaphrams with prototypical widths. Unfortunately, most sources of HO scale passenger car diaphrams provide a product that is way too wide, in an effort to close the gap. I evaluted lots of these products and in the end, went back to the diaphrams included in the Branchline® kits. Luckily, Branchline® offers these as standalone products that can be retrofitted to other manufacturer's cars as well.

So if the diaphrams are not too skinny, that means the couplers are sticking out way too far. That turns out to be the case, and in a big way! We can correct that! We can relocate the mounting post for the coupler arm without too much trouble and the big ugly gap will be gone for good!

What about the "How you gonna uncouple?" problem? It turns out that that isn't too much of a problem either. We just have to route the magnet flux lines from our handheld magnetic wand from the side of the car to a location above the coupler where they can do what we need. The high tech magnetic flux line rerouter shown in the photo to the right looks a lot like a nail. That's because it is.

The drawing to the right gives specifications for an adapter that will move the coupler back to a prototypical distance and will also put the coupler height at the prototypical 35" for passenger cars. The plate part can be fabricated from 0.030" styrene. The post can be styrene as well, but unfortunately it isn't a standard size tube that you can buy off the shelf. The dimensions shown in red are pretty critical. If you miss them by much, you'll have a noticable gap (not a big ugly gap, mind you) between the diaphrams or even worse, the couplers won't mate when the cars are pushed together. I highly recommend a set of dial calipers to assist in fabricating these adapters. Some industreous person should really provide these as cast resin parts, but for now we have to build them one at a time.

Start by cutting a slighly oversized rectangle from 0.030" styrene sheet. Then sand the edges till its dimensions match those in the drawing. I use a True Sander from NWSL for this job so I get nice square corners.

A hole for the new mounting post comes next. Not just any ol' hole either. This hole needs to be accurately located and sized. To locate the hole, first find the centerline of the part. Measure the width of the part and divide by two. Lock your dial calipers at that dimension. Draw the calipers down the part allowing one jaw to slide against the edge of the part and the other jaw to scribe a line down the part lengthwise. Now here's a trick: Leave the calipers locked at that dimension. Turn the part around and scribe another line using the opposite edge of the part as a guide. The same jaw should be sribing a line in both cases. You might wonder why the scribed lines don't perfecly overlap each other (they will if the jaws of the calipers are very sharp). What I get is two separate and parallel lines. The actual centerline of the part is halfway between these two lines (or in the middle of the single resulting line if your caliper jaws are a lot sharper than mine). Our hole needs to be somewhere along this centerline. Use the same procedure to find out exactly where along the centerline the hole needs to be. First, lock your calipers to 0.383" and scribe the first line. Next, measure the total length of the part (should be pretty close to 0.650") and substract 0.383" from that. Lock the calipers to that dimension and scribe the parallel line. What you will end up with will be a total of four crossing lines and the correct location for the hole is in the center these of lines. Use a pin mounted in a pinvise (what else?) to poke a center mark for the hole in the plastic. The photo shows the plate scribed and ready to drill. (I rubbed a pencil across it so the scribe lines would show up in the photo.) Switch out the pin for a small drill (1/16" perhaps) and drill a pilot hole. You'll need a #29 drill to get the hole to its final size. This drill is large enough to twist by hand into the soft styrene (no need for a pinvise). Hold the part on top of a scrap piece of styrene while drilling the hole to prevent the drill from grabbing the thin material. Pull the drill out periodically to make sure the hole is ending up into right place. If it's not, angle the drill to correct the problem.

Next we need to fabricate a new post. I wish I could just say go slice off a piece of size XYZ styrene rod from Evergreen or Plastruct -- but I can't because they don't provide the size we need. I used a lathe to fabricate the post from a piece of sprue from the Branchline® kit. That's why the post in the photo isn't white like the adapter plate. The post needs to be pretty close to 0.134" in outside diameter so the coupler support arm will be free to swing side to side, but have very little play otherwise. It also needs to have a #50 hole through the center so we can tap it for 2-56 screw. This is a super easy piece to make if you have access to a lathe. I have also heard of people working magic with a hand drill and a file, but my recomendation would be to use a lathe or find someone with a lathe and be very nice to them. Again, if some industreous person happened to provide these adapter as cast resin parts, we wouldn't have to go to this trouble. Maybe we should find someone who knows about resin casting and be nice to them instead!

Installing the adapter plate is pretty easy. First, remove all traces of the mounting post that is cast into the floor of the Branchline® car. I use a sharp 1/4" wood chisel (Craftsman, no less) for this task. Now test fit the adapter plate. Its final resting place should be pushed up against the body bolster of the car for proper spacing. If it goes in nicely, pull it back out, apply plastic cement to its bottom face a put back into place forever. Cement the new mounting post into the hole in the adapter plate and give plenty of time for the cement to fully cure.

Once the cement is cured, the screw hole in the mounting post should be tapped to accept a 2-56 screw. Perhaps one could simply put the screw into the unthreaded hole and let it form its own threads, but doing this is risky because the required torque might twist out our new mounting post. The super short screws included in the kit are nifty, but I don't trust them and wanted something longer. I went ahead and extended the hole in the new mounting post all the way through the floor of the car with a #50 drill so the holes would be easier to tap. I then used a 2-56 spiral pointed gun tap to cut the threads in the mounting post and into the floor of the car. These taps form threads with minimal torque and so minimal risk in de-attaching the new mounting post.

Our next step is to fabricate the high tech magnetic flux line rerouter (hereafter refered to as the bent nail). My nails came from KMart in a little 2 ounce plastic package. There's not much of a description on the package other than Nails, 1-1/2". There's also what appears to be an item number - 8675. If you are making this modification as described, you'll want to look for this same type of nail. There are a whole lot more types of nails than I ever imagined out there. This particular one has a body diameter (0.081")large enough to efficiency transmit the magnetism from the uncoupling tool, but small enough to fit between the coupler arm and the floor of the car. The head is also fairly large so the uncoupling technique will still work when the cars are on a curve and the couplers are pushed to the side of the box.

The nail will have to be modified a bit to fit properly into the floor of the car. Before we start messing with the nails though, it is wise to go ahead and modify the floor of the car to accept the nail. That way we can know when the nail is shaped correctly.

DELETE ALL THIS I searched the world over for the perfect nail. A little research revealed that I wanted a nail with a I bought lots of nails to test and I found an unfortunate curiosity. Not all nails are equal when it comes to transmitting the magnetic force -- even if they are the same diameter. I theorized that the heat treatment of the nail probably had something to box.