Most people don't fly but you can fly to Agra on Mondays with Air India (commuter division) and return Wednesday for about $115 r/t. Timings are good around mid day and if you're going to see the Taj you should also go to Agra Fort which is magnificent....so two nights there is reasonable. The flight is under an hour plus add maybe 2 hrs getting to airport and security....that might be better than 4-6 hours each way on a train depending on which train you can book.