SPRINGFIELD — For nearly an hour this week, the state’s highest court mulled a seemingly simple question: What is a riding trail?That’s because an Illinois law shields local governments from liability when injuries occur on “[a]ny hiking, riding, fishing or hunting trail.”And for 23 years, courts have relied on a line of cases saying a trail is a “riding trail” when the path runs through a “forest or mountainous region.”But a lawyer for the city of Highland Park on …