The engine currently running on the Branchline automation is a Bachmann SD40-2, UP #3450. It’s been running quite solidly and recently it developed some issues.

The issues are varied:

• Once in a while it gets stuck on the complex dual-gauge turnout next to Bear Creek. That seems highly dependent on the running speed. There’s some kind of “running” noise that I don’t remember when the engine was new.
• Whilst I was trying it at different speeds, I clearly heard some kind of mechanical “klong” a couple of times when running over straight track, so it’s not due to going over some track defect or turnout.
• The speaker’s sound is sometimes just fine, and other times has a creaking quality like the one generated by a speaker with a bad membrane. I tried to adjust the sound level, and that didn’t solve it. It comes and goes. It seems like a physical / mechanical issue.

OK time to open it up on the workbench.

Immediate observation: this is a nice design, which is actually very easy to open! There is no need to remove the couplers. Two screws hold the fuel tank, which then reveals two screws holding the entire engine hood.

Right here we can notice the cause of the sound issue. That’s a common issue with bottom-mounted speakers, they act as magnets collecting whatever metal happens to be on the track. In this case we have a tie that clearly pierced the speaker:

Here’s the design viewed from the top:

Running it open on rollers, I can feel some kind of vibration from one of the driveshafts. It seems to be the front one. Unfortunately, further exploration of the front truck reveals no obvious defect:

Update:

Upon closer inspection, it seems the driveshaft wiggles in the black U-joint plastic piece glued to the screw gear.

After consideration, I decided to remove the screw gear and the front driveshaft, leaving the engine powered only by its rear truck. The rationale was that the “Branchline is almost flat” so it shouldn’t matter. Well… that’s sort of not quite true. Turns out that the canyon section has a slight grade. Combined with the sharp curve, the engine is having a bit of a hard time when it reaches the canyon bridge. I compensated by bumping up the speed in the automation program, and that makes it usable for now.

The end goal is to get a new gear and driveshaft from Bachmann if possible to replace it and restore the front truck.I “just” need to find the right part and see whether it’s available, and of course I forgot the box with the original documentation at the museum so I don’t have it look it up at home, however luckily it’s easy to find online:

• Product: https://shop.bachmanntrains.com/index.php?main_page=product_info&products_id=6502
• The accompanying exploded part documentation lists the parts I need as H623X-00A01 + *00G03.
• Parts: https://estore.bachmanntrains.com/index.php?main_page=index&cPath=68_790_187
• Driveshaft alone:
  https://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=68_790_187&products_id=1328
• Worm gear assembly with driveshaft:
  https://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=68_790_187&products_id=7177

That last reference looks right, so I’ll probably just order a couple of these.

As for the speaker, once cleaned it produced a correct sound so I opted to not replace it yet. However, I added a mesh in front to hopefully reduce the amount of metal that it would pick up:

The pet-resistant mesh is actually a piece from a window screen that I fixed recently at home.