Как вывести ArrayList на экран

List<Track> tracks = new ArrayList<Track>();
String abbr = sc.nextLine();
String number = sc.nextLine();
String type = sc.nextLine();
String unit = sc.nextLine();
String frequency = sc.nextLine();
tracks.add(new Track(abbr, number, type, unit, frequency));
for (Track s : tracks)
        {
            System.out.println(s);
        }

public class Track // .... {
   // Fields && || Methods
   // ......
   @Override
   public String toString() {
      return new StringBuilder("Track{")
                 .append("abbr: ").append(abbr).append(",")
                 .append("number: ").append(number).append(",")
                 .append("type: ").append(type).append(",")
                 .append("unit: ").append(unit).append(",")
                 .append("frequency: ").append(frequency).append("}")
                 .toString();
   } 
}

Переопределите метод toString в классе Track

@Override
public String toString() {
    return "Track{" + "abbr=" + abbr + ", number=" + number + ", type=" + type + ", unit=" 
            + unit + ", frequency=" + frequency + '}';
}

Выводите поля с помощью геттеров:

tracks.add(new Track(abbr, number, type, unit, frequency));
for (Track s : tracks) {
     System.out.println(s.getAbbr());
     System.out.println(s.getNumber());
     System.out.println(s.getType());
     System.out.println(s.getUnit());
     System.out.println(s.getFrequency());
}

или, если у вас в классе Track нет геттеров, замените это на прямые обращения к полям:

tracks.add(new Track(abbr, number, type, unit, frequency));
for (Track s : tracks) {
     System.out.println(s.abbr);
     System.out.println(s.number);
     System.out.println(s.type);
     System.out.println(s.unit);
     System.out.println(s.frequency);
}