I tried to book tickets on cleartrip and found that they wouldn't accept any of my various U.S. issued credit cards. I then started over at www.irctc.co.in and the transaction went through on the first try.

I perfectly understand the need for effective credit card security, but limiting purchases to only those with credit cards issued in India or certain small banks --- excluding Citibank, Chase, AMEX, Capital One etc. --- doesn't provide a useful service to many, if not most, international travelers.