Wire sizes are important, yet I found it very confusing to know what sizes I should use. So I had to do some research, which I’ve summarized for myself below, maybe this is helpful for anyone else.

Locomotive internals:

• Choice: 24 AWG = 0,2 mm²
• Reasoning: PIKO uses 24 AWG for track and motor wires in their locomotives.
• Usage:
  • Velleman solid wire (10 colors) purchased from a local webshop Conrad (Belgium)

Indoors:

• Choice: 20 AWG = 0,5 mm²
• Reasoning: LGB’s track connectors (LGB 50160) have 20 AWG wire attached,
• Usage:
  • blue/red stranded wire for track (e.g. LGB 51235)
  • orange/white stranded wire for switches/signals (e.g. LGB 50130)
  • black/white stranded wire for lighting and other DC accessories (e.g. LGB 50140)

Outdoors:

• Choice: 16 AWG = 1,5 mm²
• Reasoning: LGB’s old “Jumbo” transformer (a really powerful one) recommended 1,5 mm² wires. Although 16 AWG should be enough for most railways outdoors, you could argue to go one step larger at 14 AWG.
• Usage:
  • blue/red stranded wire for track (e.g. PIKO 35401)
  • orange/white stranded wire for switches/signals (e.g. PIKO 35402)
  • black/white stranded wire for lighting and other DC accessories (e.g. PIKO 35400)