When halt is more d loco which has more acceleration is alloted to gain d speed. EG: Earlier 12608 / 07 - Lalbagh was alloted WAP4 when it had only 2 stops. i.e. BNC / KPD, it used to cover SBC-MAS within 5 Hrs. now it has about 11 stops. So to gain d lost time in event of increased stops, it has now been alloted WAP7.
Rgds
Kamesh L Krishnamoorthy
SBC