I have two of them, an original run and a new run. The original run will 'click' slightly when reversed and then settle in, also tends to click a bit on sharper curves. Overall, it runs better than the new one, clicking or not.
The bigger question here, seriously, is if it is actually binding or just clicking. Because if you really study the drive, it's a wonder this thing was ever built at all as a model. It's a diesel with ride-along driveshafts. The problem is threefold in the drive system:
1) There are only two axles - one on each truck, that actually engage the driveshaft. But they are on opposite trucks. The fit on those is very loose, and the gauge on the wheels is also rather haphazard, if you study it, so that the clearances between the bevel gear and the truck face are quite variable in assembly.
2) If you get the trucks off and 'disorient' them so that putting them back on is even a little bit off - you can introduce torque against the wheels through the driveshaft; i.e. what used to be free running is, in some cases, now fighting itself through the driveshaft.
3) If you introduce enough torque through the drive system fighting itself, the first thing to go 'POP' tends the be the universal at the rear end of the cylinders, sending small pieces into the scenery or floor.
So unless you're really prepared for some mental stress, don't mess with these if they are running OK other than a soft clicking. If they are hanging up and clicking, different issue - you really do have to address that, particularly if you've already tried to take the trucks off, put them back, and now it's clicking.
I think the clicking is primarily produced by the two truck face bevel gears 'fighting' on the wheel faces. If you study it, you'll see that the wheels are plastic on a single shaft axle. You 'maybe' can get tweezer points in there and push the one wheel face out from the truck frame a hair to tighten up that gear fit, but now you've widened the gauge and have to possibly move the other wheel over as well, which may skew the gauge in the truck. But all bets are off if you decide to either pull the truck out of the frame or pull the wheels, you may make this much worse rather than better. I've had my driveshafts pop several times and it's one of the most stressful repairs I've ever done in N.
One thing I've done is check to make sure - with tweezer points and gentle testing - that there is always slight 'slack' in the driveshaft train at all times. You ought to be able to wiggle it just a hair to prove it's not binding, just coasting along as it should. Adjusting that, if you think of it, is literally dropping one truck and rotating the spur gear ONE TOOTH, compared to the worm, reassembling it, and testing it slowly and gently to see if what you did bound it up more or introduced the slack it needs. There's actually a lot of slop in there to work with, but you can still take off a truck and put it back on and take all of that slack out without realizing it.
Ideally, I don't know why they put gears on both ends of the driveshaft trucks AND a non-slip universal with a square shaft. If just one of those universals was round, instead of square, each truck would drive and spin half the works and they two sides couldn't fight the middle as can currently happen.
I've made all kinds of modifications to mine but I'm also hyper-careful and very gentle with it. The first time it shed the driveshaft universals in a sea of Woodland Scenics polyfiber balls I thought I'd cry. After I got it to run well again I vowed never to mess with the drivetrain unnecessarily!
