I have been making and using similar tools for about 30 years now, but using a lathe and mill to do so.
Ric's photo shows a way forward for those of us who do not have lathes.
Apologies for the length, This was ment to be short, but is fast approaching book length. I hope some find it useful.
A brass tool will cut plastic, both Delren and most polymers used in 3-D modeling, with occasional resharpening. Steel will last longer, but is a little harder to work with. Use 1/8" stock if you are turning with a Dremel, either Ric's 3/16" or 1/4" stock if turning with a drill. Cut your piece a little longer than the width over axleboxes, chuck it with a washer to protect the chuck, run at moderate speed, and file your point.
The point angle needs to be a little blunter than the axle points. For rounded tip axles (Kadee) the tool point needs to be a little blunter than the taper behind the axle point. The idea here is to ensure that the truck rides on the axle's point,without any contact elsewhere. When filing a point this way, there is a tendency to make it sharper than wanted, because the tip, at zero radius, while rotating is not actually moving. So start with an overly blunt point, stroke the file, and work your way back to the desired angle. A coarse file can be used initially, but finish up with your finest single-cut file, always using a file handle. It will help if whatever you use to rotate the work is solidly mounted, so you can control the file with both hands. A heavy drill press is ideal, but I have used a Dremel in their drill press, laid on its side
For all materials the cutting edge should be square to the tool's cone's surface. This can be either a groove, as Ric used, or a half diameter flat. The groove is safer for just cleaning out a well molded axlebox, as it provides a larger guide surface. A groove for cutting right handed, when viewed from the side, point up, should pass just to the right of the point. The half flat version is actually a fair drill, and can also be used to deepen axlebox bearing cavities, but will serve for cleanup if used with light pressure. this will cut in either direction.
Ric cut his groove with a cutoff wheel in a Dremel tool. I would want either the tool or the Dremel to be solidly mounted to something, so I could hold the other with both hands for control. As always with cutoff wheels, wear safety glasses and keep your head out of the wheel's plane of rotation.
For a flat cutting edge, file the flat to almost full depth, and finish on a sharpening stone or flat diamond file. Here one desires to avoid any hint of rounding at the flat's edges, creating a square edge that is actually quite sharp. If I were making another, I would use a double ended pin vise, and rig a guide to the back end, to prevent the pin vise rotating, while finishing the flat. A piece of heavy wire, bent into a triangle perpendicular to the vise's axis, and clamped in the second collet, should suffice.
Having got your cutting point, trim the tool to final length, and turn the guide cone end, slightly rounding at the tip. Giving tht cutting is done when pressing on opposed axleboxes, the afctual length is not critical, given the spring of the side frames. A little short is probably better than long, minimizing insertion stress on the sideframes. Insertion stress may be further reduced by truncating the guide cone. Excessive variation of axle length, between manufacturers, may make multiple tools desirable.
To use, make a cleanup spin under light pressure in each of an opposed pair of axleboxes, then insert a wheel set. If the fit is at all tight, give the axleboxes another pass with slightly greater pressure. Continue until the wheel set spins freely, with just a hint of side play. With these conical bearing surfaces, minimum side play is needed to ensure consistent center plate height for any any pair of trucks. While checking for spin, make certain that the wheels are not dragging on the inner axlebox face, brake shoes, or any untrimmed molding flanges on Delren trucks.
If the fit is loose initially, one can stop with the cleanup and use as is. Such trucks should still roll well, but they will have some side slop, which may actually help rigid trucks get over rough track. Otherwise one needs longer axles or a shorter bolster. NWSL axles are the longest HOn3' axles available, at a nominal 0.75". Slightly longer HOn3'6" wheelsets may be available from Australian sources. My preferred way to narrow one piece truck frames is to splice bolster halves with a piece of K&S brass channel, or square tubing. Cut and file the bolster top flat, cut out the center plate, and cut the brass splice to the required length between side frames. If too loose a fit over the bolster halves, add styrene strip fillers, but the bolster halves should be free enough to twist slightly, equalizing the weight on wheels. Drill the splice sides at the two quarter points, about half depth on the bolster halves, then insert the bolster halves and drill through them, opening these holes slightly with a tapered reamer. Drill the splice centrally for your mounting screw, insert the bolster halves and glue bits of wire in to loosely retain them.
It is probably best to paint trucks and wheel sets before cleaning axlebox bearing surfaces and fitting the wheel sets. After fitting, remove wheels and add a drop of well stirred Neolube, or other colloidal graphite, letting the alcohol evaporate before replacing wheels. For painting wheel sets, I use a holder made from chopsticks. Start with an un-split pair and drill a pair of guide pin holes through the splitting plane. Split, glue guide pins in one side, add a rubber band for compression, and drill axle end holes in the other end, This holder is also useful for cleaning paint from plated brass, nickel-silver, or zinc die-cast (Kadee & MT) wheels, using a brass brush wheel in the Dremel.. But do not be tempted to use the wire brush to spin Kadee wheels wile cleaning them - I have meltted axle ends.
The above will suffice for plastic or tin alloy side frames, or for just smoothing brass sideframes using a steel tool. But for cleaning occasional brass bubble out of brass axleboxes with this type tool, one needs the rigidity of a mill, or a drill press fitted with X-Y table, as well as fixtures to clamp and hold the individual sideframes securely, while working on them. With hand tools this is probably a matter of using micro chisels to pare down the bubble. I am not aware of any source for these, but one might make them from high speed drill shanks, mounted in small center punch size holders, Work holding fixitures will still be required. I have not tried this.