Touching my ass while passing by

I early twenty, and was looking at the timetable at platform 2. Platform 2 is a very wide platform and it wasn’t crowded at all when it happened. While I was looking, I felt something like a finger or part of a hand gliding across my butt. I was surprised that I had any physical contact in such a quiet area, so it took some time for me to process what exactly happened. And when I looked up, the person was already out of sight.