Does anyone know of a small wire for lighting that is stiff so to look like piping?

I would use a piece of small 'enameled' wire from a transformer and thread it through a small brass tube and then use black heat shrink over the brass tube. this would give you 2 conductors to the light.

yellow_cad said:

wire with black insulation (or could be painted black) that is stiff

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Any solid core, not stranded, wire will be stiff. The Romex stuff electricians use for house wiring is solid and I often use it for piping.
If you want something that looks thin, go for 18 gauge or smaller (higher number.) Amazon has 26ga enamaled copper 'magnet wire', and solid black 18ga 'hookup wire'.

Pete, at the same Amazon link you have for 18 ga solid wire, they also have 20, 22, 24, and 26. Do you find 18 ga as the smallest diameter that is effective?

What span do you want to bridge?

You can also get already straight aluminum or stainless steel rods all over the place. Doesn't need to be copper to conduct.
Straightening copper wire can drive you mad

yellow_cad said:

find 18 ga as the smallest diameter that is effective?

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What does effective mean in this context? Thick enough to conduct power to something? Strong enough to stay in place? I picked an arbitrary number, as the Romex is usually 14ga plus insulation so it is way too big for our trains.
I suggest you figure out what you want the wire to do, and then what size pipe it has to represent and then you'll have an answer?

Just want to run the wire to look like piping from the headlight to the cab of a 20.3 steam locomotive.

figure the scale pipe in inches... divide by 20.3.... then look for wire in that diameter... (you can look up wire dimensions on the Internet)

yellow_cad said:

like piping from the headlight

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I used hypodermic tubing to replace the handrails, the stuff they make needles from. You can buy 3 ft lengths on Amazon. Measure the diameter of the handrails and buy the appropriate size. Then buy some wire small enough to slide inside - the magnet wire will work. At the front, you can file an opening on the inside of the tube to bring the wire to the headlight.