Is it wrong to walk on the dirt by railroad track to drop off flowers for a lost friend?

You've already answered your own question by admitting you knew you were tresping. Sorry, you violated their rights to do something that you wanted/needed to do. When people break the law and get hurt those not at fault (the property owners) still can & do get sued. The RR Cop sounds like he was a jerk but even with an adult present people do get hurt. I had a relative crippled for life in a railroad yard & through friends in Fire/Rescue have heard of numerous homeless people killed while walking alongside the tracks. Be careful.