I think what they are asking for is an account (true or fictional) of a time that you have been in a waiting room at a station observing people. If you imagine that scenario, it's pretty easy to come up with an interesting essay.
A major city is the best place to observe human behaviour as there will be people from all walks of life and different nationalities. A local, urban station will likely have similar people but even so, they will differ - old, young, friendly etc.
The person who answered before me is a classic example actually. She gets nervous and won't make eye contact so you could imagine that she is one of the people in the station, and describe what she is doing. She hasn't looked at anyone and is busy on her phone.
Some people get nervous and anxious at stations so there might be someone looking scared or pacing up and down. There might be a mother with a young baby who is crying. Are people disturbed by this? Do they look annoyed or sympathetic? I'm guessing there will be a mixture of reactions.
A friend of mine recently experienced a man dressed in a suit standing on the platform waiting for a train, he looked normal but then my friend noticed he wasn't wearing any shoes or socks! Someone like this is always fun to add to the scene.
Maybe there are people talking in a different language or wearing clothes that differ from what most people are wearing.
Someone might have a suitcase which could make you wonder where they are travelling to. Are they happy? In which case they might be going on holiday or visiting a loved one. Are they looking anxious or upset? Maybe they have left their partner or are visiting a loved one who is ill.
It's just a case of using your imagination really, I'm sure you can come up with something interesting.