I setttled on using soft silicone wire, 24-26awg for power connections, 28-30awg for lighting.
As a result, I highly recommend this type of wire for BPRC work.
I list some specifications below and suggest you check see if it's right for you.
I should also mention I prefer using a 2S arrangement (2-Lithium-ion cells in series plus a 5V voltage regulator (for my 6V Swiss motors),
or a 9V voltage regulator for stock 12V or new can motors (efficient up or down from 7.4V battery output).
I use 32 gauge Silicon Wire.
I find the smaller diameter helps with reducing volume.
It is surprising how much space wiring takes up.
I use three colours. Red, Black and Yellow for the balance charging.
I have made up the 2S battery configuration with a Pololu step up to 9v.
This works quite well on the OEM motors on Joe Works and similar 1980s NG locos.
____________________ Rod Hutchinson
I find a 1S lipo with a 5V or 9V voltage regulator works well with locomotives needing up to 0.3A.
For larger power requirements the nominal 7.4V from a 2S battery is good up to 12V.
I have about a 50% mix of 1S and 2s battery packs and 1 or 2 12V regulators plus some 5V ones.