Sorry, that's not enough time for travel. From Nizammuddin station, its about 35 km to the international airport. Assuming a regular weekday, you are looking at a minimum of 90 minutes. If you want to get a pre paid taxi and you assume your train will be on time (trains routinely run between 2-12 hours late this time of the year, thanks to the heavy fog blanketing the Delhi area - early January) - I would keep 2 hours as a minimum. Still, its India and honestly, you really cannot plan.

Flights to the US normally depart around midnight or later. I'd say, if this is the case, you are comfortable. But check a couple of things in advance
- does the train originate at Sawai Madhopur?
- can you get off one stop prior to Nizammuddin? If so, does this work out to be closer to the airport, bypassing Delhi traffic? (especially around South Extention and the Ring Road areas?)
- can you keep a back up rental car ready, to drop you to Delhi IGI airport?