This is the 14995th most frequent Turkish word.
seferlerinin
"Seferlerinin" means "of the journeys/campaigns" or "of the expeditions" in English.
In this sentence, 'seferlerinin' refers to the trips (seferler) belonging to or related to the buses, indicating possession.
Otobüs seferlerinin saatleri sık sık değişiyor.
The timing of the bus trips changes frequently.
Here, 'seferlerinin' specifies the trips (seferler) associated with trains, further emphasizing the genitive form to denote frequency of action.
Tren seferlerinin artırılması gerektiğini düşünüyorum.
I think the frequency of train trips should be increased.
In this context, 'seferlerinin' pertains to the trips (seferler) of airplanes, indicating ownership or association.