Take the Gatimaan Express. It leaves Nizamuddin Station at 8am and reaches Agra at 9:40 am. Negotiate with rickshaw at 100 INR to reach the Taj West Gate , then walk up to Taj Mahal.
On the way back, come out from the West Gate. Visit Red fort which is at walking distance and the waymarkers show the way too. From there, come back to station at 100 INR in rickshaw. Return Gatimaan Express leaves at 5:50 pm and reaches back to delhi at around 7:30 pm…
Tickets are available a day in advance. Train ticket with meal is 750 INR and without meal is 585 INR. It’s a very simple meal. There will be least chances of Gatimaan Express and Shatabdi Express getting late.
If you are not Indian, then you can use the Foreign Tourist Ticket Counter located on first floor of New Delhi Railway Station.