It might seem counterintuitive to want trucks to have friction but on a small switching layout it can add a wonderful sense of realism. This is a small modular switching layout that has relatively few cars on the layout. Trains on the layout move slowly and need to stop and start realistically. A little friction helps make the cars move like they carry the weight their full size counterparts do.
To add friction, I superglue some fine phosphor bronze wire to one of the truck frames. The wire needs to be far enough away from the wheels but also not too close the the bolster pin to make it bind on curves. I find a 1″ piece 0.010 wire works well, I superglue it in place and then trim off any extra wire. I always worry that the wire will pop off and cause a short but I’ve never had a problem since its under pressure from both axles. This piece of wire adds enough friction to keep cars in place if the modules are bumped, it also helps keep the slack out of the couplers in slow moving cars.