If you are thinking about morning travel to Agra then there is a daily 6am Shanbhati Express train on Delhi to Agra. It should take about 2.5 hours and is one of few trains that runs on time as it ferries the tourists from Delhi on the day trips to Agra. It all "chair class" and is as respectable as it gets on Indian trains (or so I 'm told). It is more cilivised that sitting in a car for 4 hours.

You might want to consider using a local reputable travel agent before you arrive in India (PM me if you are interested in one) to get the train tickets for you, and ferry you to the Taj Mahal. The guide should then take you to see the Red Fort and the little Taj which are the other two sites to see at the Taj Mahal. The honest truth is there all the tourist on this train will be heading to to see the Taj and there is likely to be a bit of a mad dash.

My personal suggestion is to see the Taj first thing in the morning before the on-slaught of tourist starts. That means wandering down there about 8am. Depending on the time of year, the changing angle of the sun gives the Taj different shades. There is something beautiful and mytiscal about the Taj if you get to survey it in a peaceful surrounding.

A friend of mine visit the Taj in the afternoon amongst the crowds and massive queues to get in and did not enjoy the experience at all.

I would also recommend the Oberoi Amarvilas, pricey but all rooms have great views of the Taj (subject to mist, fog, pollution, etc) and the amenities are fantastic. The hotel also provides a free golf buggy service from the hotel to the gates of the Taj) (no cars allowed near the Taj; its electric buses, horse and cart or even camel and cart).

You might want to consider travelling down to Agra late on Saturday morning (after a rickshaw trip throught the spice markets maybe), enjoy the hotel facilities in Agra on Sat afternoon. I understand that sunsets are great as well too and most tourist leave to return to Delhi before this. The standard tour of Agra (Taj, Red Fort and Little Taj plus a stop to a factory) starts about 8 or 8:30am and finishes around 1 to 1:30pm. That leaves ample time for a car to take you back to Delhi with plenty of time to make your flight. Not sure about train times on the return leg. Not sure I would risk a train if there is a flight to catch.

If you are travelling down to Agra late morning/early afternoon from Delhi, you might want to visit Humayun's Tomb in Delhi which is not only a World Heritage Site but form the blue-print for the Taj Mahal (there are 2 generations of king apart) to compare and contrast.

I would really advise against using a guide you find at the train station. You have no idea what you are letting yourself in for given the limited time you have in Agra.