You are eligible for both foreigners quota as US citizens and regular quota as NRIs... If you want to book in regular quota, provided you have somekind of an Indian Photo ID card or any Photo Credit card, you can book it on www.irctc.co.in from USA itself. Else you have to visit a near by PRS to book under foreigners quota. Foreigners quota would generally be available until 2-3 days before the journey unless completely availed by some foreigners. If you are booking through irctc book as early as possible as it is meant for all kinds of passengers.