It seems to me that too many people use wire that is overkill in terms of thickness leading to shoehorning and connection issues particularly in small places. The same thing happens in home audio with massive expensive cabling that has shown over the years not to make a difference in terms of sound.
Here is what Lionel says:
Wire comes in many sizes in solid and stranded varieties. Stranded wire is really a bundle of smaller diameter wires wound together. Wire size is indicated by a number – the larger the number, the smaller the wire. Most house wiring in No. 12, solid wire. For your layout, we recommend a No. 16 stranded bus wire with No. 18 feeders. The feeders do not have to be as large because they are not as long and it is easier to attach the smaller wire to the tracks. Stranded wire offers better electrical flow (electricity flows around a wire, not through it) and it is easier to bend. We recommend adding feeders at least every 10 to 20 feet around the track.
For lights and other low amp accessories, even smaller wire can be used on the feeders. Lighted accessories should use No. 20 or 22 wire.Phone cable works very well for switch motors. It is color coded, easily available and cheap!
John