I agree that there is not a "short trip" option to Agra. Travel by car is over small roads with lots of unimproved roadway. Interesting to do, but you will be worn out, especially if you are fighting jet lag. 5-6 hrs. is about right, if I recall. If it's your one and only chance to see the Taj Mahal- go for it. Just realize what you are getting into. The train is better, IMHO. Either way, plan for a long day. It's worth the trip, though!