Regarding joining track saw rails, I stress I do not have a track saw, and therefore no rails!
But as a reasonably competent "metal basher" I wonder why people don't make their own joiners. I SUGGEST the following which only needs a reasonably accurate capability with a hack saw and a file or two:
1. Select a piece of sheet steel of the same thickness as the inside of the "U" channel. The joiner should slip snugly into that channel. NO plastic please (I THINK some of the plastic joiners are plastic. I don't care what sort of plastic, IMO that type of joiner cannot possibly withstand the bending moment put on it when you lift 2 rails joined with those things for very long);
2. Hacksaw a piece off your piece of chosen steel. IMO, it should be about 30 cm (12 inches) long. To me, ALL the shop-bought joiners I've seen in vids such as Peter Millard's are just too short! For the same basic reason as in point 1. above;
3. ACCURATELY cut and file your 30 cm piece of steel so that it is a SNUG sliding fit into the "U" channel. Note that due to the nature of ali extrusions, the corners of the "U" slot are going to be slightly radiused. You'll need a fine file to SLOWLY and carefully copy those radii on to your steel joiner. Again, a SNUG fit is what we're after;
4. Drill and tap 4 holes into your joiner, suitable thread size for the "U" channel. I'd guess about 4 or 5 mm, perhaps 6 mm (?) will do it;
5. Position those 4 holes so that the extreme LH and RH holes are within, say, 10 mm away from each end of the joiner. The other 2 holes should be positioned so that they are, I'd guess, 10 mm each side of the exact centre line of your joiner;
6. Choose bolts with head type so that when fastened into position, the head doesn't foul the passage of the saw along the rail. I guess that means C/S (but repeat, I don't own such saw or rail/s, so am not even sure if the bed of the saw plate does indeed pass over the heads of the joiner bolts. You'll have to check with your own particular saw/rail combo). The LENGTH of those 4 bolts will be critical (as measured when the bolts are driven fully home). That's easy. Just insert any nut onto a joiner bolt, keep checking/testing, and keep filing tiny amounts off until the bolt tightens the joiner on to the rail without any distortion. Then copy that length onto the other 3 bolts. BTW, putting a nut onto the bolt to be shortened before cutting/filing to length is simply to ensure that there is no "rough rag" on the end of the bolt, as removing the nut automatically restores any thread deformation. Perhaps a light touch with a needle file will finish off any remaining ragged thread portion. Make sure that the cut end of each joiner bolt is dead square to the axis of the bolt. That way, when tightened, the whole bolt end will bear evenly on the bottom of the "U" channel.
If all that above sounds/reads long-winded, I'd guess that the average user needs only one joiner, max two.
I'm sure I could knock out such a joiner from scratch in about 30 mins. If not used to metal bench working, you may need a bit longer, but A) it's a good way of improving a useful hand-skill if you don't already have that experience, and B), without KNOWING, I "reckon" that the above home-made joiner will produce rail joiners which will be long-lasting and produce a stiffer and more accurately aligned joint between two rails than any of the shop-bought joiners I've seen in videos, etc.
I may be wrong - "your mileage may vary", etc, etc. Offered FWIW