aap +1 mein drag force ke baare mein pade honge..1/2(ρAv2)...high speed rail mein ye force ka component pantograph ko neeche laneki koshish karegi.....contact bane rakhne keliye aapke paas kuch na kuch feed back control chhiye.......contact sensor lagana mushkil hota hi....yisliye instead of active feed back control...they use passive feed back control and approximately calculate pantograph mechanism joint forces required for a given speed of train to counter the drag force...this nasty feed back control when incorporated into a normal panto becomes so called high speed panto....