Walthers makes HO-scale couplers under the name "Proto MAX". They are only available in a pack of 20 pair (the 4-pair-pack has been discontinued). The product ID is 920-6002. In general, these couplers are larger than the Kadee #5 couplers and smaller than the Kadee #802 couplers, which are the other couplers that most S-scale modelers use. S-Helper Service designed their own coupler, because Kadee wouldn't do a custom-run for them and their coupler box was too big for some of the S-Helper Service cars (the ore car, for example). MTH continued to manufacture those couplers when they took over S-Helper Service's tooling. We will have to wait and see if Scale Trains is going to produce those couplers now that they have bought MTH's S-scale product line.
(external link: Proto MAX Couplers)
I have measured these three couplers and found the following measurements:
Coupler Face Height:
Prototype = 11"
Proto MAX = 10"
S-Helper Service = 11.5"
Kadee #802 = 12"
Prototype = ?
Proto MAX = 15.5"
S-Helper Service = 17.5"
Kadee #802 = 18"
Subjectively, when I compare these three model couplers, I find the Proto Max HO-scale coupler to be of a more pleasing size. I find the S-scale couplers, visually, a bit too large.
This photo shows the four parts that make up one Walthers Proto MAX coupler. The metal centering spring fits in the main body at the bottom of the photo. Then the coupler is installed, and the lid closes the whole assembly. The parts need to be held together at all times until the mounting screws are installed, because the lid doesn't snap onto the body.
There are several things I like about this coupler. First, the entire head is made of diecast metal, not plastic. It looks like the real thing. Also, because of that, it should be able to stand up to the forces of S-scale equipment.
The other thing I like, which is a personal taste, is that they are painted black. I know that the brown couplers are supposed to look like rusted couplers, but to my eye they don't.
The final issue which made me decide in favor of this Proto MAX coupler is that they are easy to uncouple. My preferred method for uncoupling is to insert a round, pointy BBQ stick (large toothpick-like) in between the couplers, twist, and the couplers come apart. When I initially started using the Kadee and S-Helper Service couplers that come with the rolling stock, I couldn't get them to uncouple using that method.
Of all the praise I have for the Proto MAX coupler, there are a few things I don't like. There is some play between the back of the coupler and the coupler's face, which means that you can actually run out the slack on a train (that doesn't apply to the Kadee and SHS couplers). I don't think that this causes the jo-jo effect that a train full of Kadee couplers produces, but it can cause some motion. I think, however, that because of this spacing, uncoupling is easier, so, I am more than happy to trade that off.
The trip pin is way too short for S-scale equipment. However, I never use magnetic uncouplers; I prefer to manually uncouple. Therefore the trip pins are cut off on my couplers, and so this is a non-issue. It might be an issue for you if you want to use these.
Finally, the biggest problem I have found with these couplers is that the mounting holes on their draft gear box are just a hair too far apart to be an exact replacement of "normal" S-scale couplers in SHS rolling stock. So, I just use the center hole only for the screw, and then add a single, small drop of superglue on the back of the coupler draft box where it attaches to the underside of the car's body, so that it doesn't rotate. If you have a kit that doesn't have pre-drilled holes, or you are scratchbuilding a car, then that is not an issue.
Once installed on two cars, they still leave a 3-foot gap between the cars, which is prototypically correct. I have had no operational issues with them being used in a full-length freight train on our club layout, so they are plenty strong for S-scale use.
This photo shows the absolute tightest curve on my previous layout, and the couplers seem to be able to handle it just fine. One final comment about comparing these Proto MAX couplers to Sergent couplers. The S-scale Sergent couplers look like the real thing and are correctly sized. The Proto MAX coupler has about the same 3D volume space as occupied by a Sergent S-scale coupler. Their shapes are different, but they are about the same size.