The Spectrum Dash-8 40C has a reputation as a poor performer.  One of the layouts where I operate frequently had a few, and their drives all finally disintegrated from use.  One survives as a dummy unit.  I spent a bit of time trying to smooth it out but to no avail.

I got a Spectrum GP30 when those first came out, and detailed the shell in anticipation of being able to tune up the mechanism.  I never could stop the thing from vibrating at low to medium speeds - just enough so you always heard couplers rattling on cars it was coupled to.  I've been able to fine tune other mechanisms to run very smoothly and quietly, but never could quite get a Spectrum to deliver.

Rob, thanks for the info!  I kinda figured the best thing to do with these units was to turn them into dummy units.  I have one done already and they do make for a smooth and efficient roller!

I also found that the Spectrum 6 axle units were not of the caliber I expected.  Biggest problem is in the gears in the trucks.  Small, narrow and and prone to damage.  Pull all the guts out but keep the electrical contacts. At least you can still have lights and additional power pickup for you powered units.  Older Blue Box Athearns were and still are far superior in there mechanism than those Spectrum units.  And they are repairable.

The units are definitely not up to the caliber that most units are these days!  But luckily, I picked these units up dirt cheap and they will make perfect dummy units.  The detail out of the box is pretty decent and with a few added detail parts they will make good B units.  Just gotta find the time to take the shells off and pull all the guts out! 

I've found there are variations in Bachmann's 4-axle Spectrum drives, and despite them using the same motors, gears and drive shafts I haven't been able to narrow down what makes one operate better than another.

I managed to pick up a decent running GP30 from their first run, and used the drive to create a scale-width GP35 (the Kato model hadn't been released yet). It is definitely louder than the top-of-the-line current production locos (Atlas/Kato/Intermountain/etc) but it runs smooth even at low speeds. However, I've also purchased a couple more GP30's afterwards that are like coffee grinders by comparison. I was unsuccessful in getting these to run smoother, and they've since been relegated to "stored unservicable / parts source" status.

Another good runner I managed to aquire is a Spectrum F-M H16-44, it's not only a smooth runner but quiet too. It is on par with Kato-drive quality (seriously) yet the inards powering the thing are the same as those GP30's. Go figure?

With the older 4-axle Bachmann Spectum units, all I can conclude is that it seems to be hit-or-miss as to how the things operate. If you can, you should give one a test-drive before purchasing from any flea markets.

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Now as for the 6-axle Spectrum C40-8 models, I've never purchased any since I model an earlier era. However a couple of friends who are modern-era modellers (one models UP, the other CR) did obtain a good number of these when they first came out (especially the UP guy). Both have long since retired them once the Atlas models were released. As you wrote; the detail on them is acceptable, but no amount of tinkering with these drives can get them to run much smoother. As Nelsonb related earlier, there's something with these smaller/narrower gears in the old 6-axle Spectrums that causes this.