4yo's obsession

My 4yo boy is recently obsessed with escalators, travelators and stairs. He is like a spolit recorder, wanting to see a certain broken escalator near his sch everyday. Then, he would go on to say that the escalator needs gear in order to move and it cannot be repaired now as there is no spare parts. He had been repeating the exact same thing for the past one mth or so! Back home, he would request to view photos of escalators and travelators, and make us go thru one by one with him if the photo is showing an escalator or travelator. While it's good bonding, it gets frustrating when we need to repeat the same thing so mny times zzz Any of ur kids obsessed with anything? Usually how long will they take to grow out of it?