WTT enlists technical happenings which if dont happen will follow PTT as usually Pass Thru/Overtakes/Track/Freight-PF Clearance arnt strictly followed & the same holds true for Parcel!!
Usually slack exist in PTT (taken as WTT) but that usually is not enough (recent 5 mins enhanced stoppage for Parcel with slacked ARR ex BCY or BSB..train will not wait WTT but exact PTT departure if no Parcel things happen ex BCY!! Train can overshoot WTT but PTT!!