Years ago someone showed me a neat and inexpensive way to plant invisible but highly effective uncoupling magnets under track. I have used this many times in N scale, but HO should work similarly.
The ends of Atlas sectional track have a little recess under each rail; when you connect two sections, these form a little rectangle, which in N scale happens to be the perfect size to accept a 1/8" diameter rare earth magnet on each side. These are sold at Radio Shack in packs of 2; bulk quantities and other sizes can be ordered here. (Again, I haven't tried this in HO or other scales but I'm guessing the 1/4" diameter magnet will work for HO, not sure about the proper thickness though).
Once they're in place, you can move your couplers directly over this track section, and voila, the knuckes are pulled apart. And, no unslightly bar magnet in the middle of your track.
Bear in mind that in order for this to work properly, the couplers must be very close to where the magnets are, and both couplers must be straight. This means that if either or both sides will be a locomotive (or a car with body-mounted couplers), then the section of track on that side of the magnets needs to be a straight section. Cars with truck-mounted couplers are more tolerant if the section is a gradual curve.
Try it, you'll like it!