The main reason for using heavier gage wire is to prevent a lot of current loss on long wire runs. Once you get to the turnout machine itself, the thin wires of which the solenoid coils are made are short enough not to be a problem. As was posted, if you have a bunch of the 18 ga. laying around, and 24 ft. runs from the control panel, then, by all means, use the 18 ga. Otherwise, 22-24 ga. will do just fine. Feeder wires to the track is another story. You aren't going to be able to solder 18 ga. to the track very easily. Again 22-24 ga. will probably do from the track to a bus of heavier wire. Keep the "pigtails" that are soldered to the track as short as practicable (probably around 6-inches. These can then be soldered or suitcase connectors used to heavier gage wiring to the control panel. It is the long run that creates increased resistance, so the short leads shouldn't cause any problems