A few suggestions - to get the juices flowing

I keep some computer mouse cable-- the fine wires within sometimes do wonders, but I think motor power would be too much.

I sometimes use coffee stirring straws for round shapes, but not at all flexible

and where wires cannot be disguised, I braid them together..  think of the signal wires along the right of way in congested areas. Not a true braid, but a tied up bundle.  (Around here, clamped against the retaining walls.)

Spray bottle tubing

Small spray bottles are a good source of small-diameter tubing. Scent sprays or medical sprays are maybe the smallest. If the empty bottles are about to be thrown out, see if the tubing would be useful.

Try the model aircraft/model car hobby area

Hi Matt,

have a look at the fuel pipe used in I/C flying model aircraft and I/C model cars. It comes in a range of diameters.

Small diameter "unshrunk" heat-shrink tubing might be another option. It is thin walled and it should have enough "give" if left "unshrunk" 

Happy hunting

small diameter flex tubing

You could try 3mm braided sleeving. Might do the trick.

wire insulation

another member suggested stripping wires and saving the insulation for this purpose, lots of different sizes and colors

Take a look at

some small diameter shrink tubing.  It will be more flexible if you do not shrink it, but you might try it either way.  Another idea just came to mind.  Find a small black wire and pull the copper out leaving the insulation.  Cut the insulation into 1/16 inch lengths and slide each slice over your wire bundle.  This will be very tedious and take a while.  Put a drop of ACC on the first and last slice. This will be the most flexible approach I can think of.  The water hose from the tender is is a pretty large diameter.

Do you have a photo

of what the finished product should look like? Seems like Anything  you put small wires thru is going to have to be a fairly loose fit to get them thru .You might try a flyfishing supply , I've gotten various small cords and such from them. I recall some small flexible plastic tubing on a roll that I used for O scale brake hoses....DaveB

It does not have to be a

It does not have to be a solid tube, rather a contiguous link of tube could conceal the wires too. Suppose you took the appropriate size of shrink tube, but cut into small 1/8" or 1/16" segments, and slid over the wires. Do not bother to heat shrink them, just let them touch like beads on a string.

Use the wires themselves

Attach the wires to both loco and tender (with some slack), form the wires into a “drape”, or sag, between units, and paint black.  If you put three per side, the number of wires shouldn’t be obvious.

Felxible tubing source

Also try here:

https://www.mcmaster.com/tubing/flexibility~very-flexible/flexibility~ultra-flexible/flexibility~continuous-flex/flexibility~flexible/

Flexible Wire

You didn't mention what scale you are modeling but Adafruit has silicone covered stranded wire in small diameters that may be suitable. The silicone jacket is more flexible than the usual PVC or polyethylene covered wire.  (https://www.adafruit.com/?q=silicone+wire&sort=BestMatch) ....BobB

"Flexo sleeving"

Do an on line search for this term. It is for 'bundling solutions' and can be found down to 1/16in dia. If 1/8in black will work for you, I can put some in an envelope to mail to you.  You may find 1/16in in an electric supply store.
If you have never seen it, it is braided and acts like a "Chinese handcuff", if you remember those you would put on your fingers. 

It's used for bundling wires, but also for stress relief when wires are suspended. You may have seen them when very large 200A cables hang down to a machine and need support.
I can send pics, if you like... just need to get them out of my phone first, and that will wait until tomorrow.
Best regards,
Douglas in N TX

"Flexo sleeving"

Flexo Pex w/HOoupler