Agar technically dekha jaye, to Agar kisi passenger train ka timetable aggressive ho, then usme P7 dena hi achha hai, rather then less stoppage SF/M EXP, jiska avg speed more than 55hai wo bhi, kyunki wo to P4 me bhi achha bhagega..lekin agar 50+avg speed wale all stoppage passenger train ho, to usme P7 achha hai
But ye technical baatein, otherwise 3phaser to premium trains ke lia hi banaya gaya hai :p