Day trip to Taj Mahal - tour recs?

If you can get a work colleague to accompany you that's your best bet. Unless it has been recommended by someone you know and trust it is not a good idea to use any service that you are not familiar with. There are scammers scamming the scammers at the Taj.
WRT entry, if you enter from the back entrance the lines will be much shorter. Here a guide that can be trusted to get you around will make all the difference... early is better.
The drive there is not particularly eventful but when you arrive in Agra the traffic becomes pretty intense....Pay close attention to where you are and don't get too far off the beaten path.

Thank you Mack!
I had hope that someone here could recommend one they took and then I could research that but what you say makes sense. I did find the train info and it looks easier and quicker than messing around with cars or relying on someone else we don't know to get us to Agra. Thanks for your insight!

It's been a few years but my situation was similar to your own, in that I was scheduled to be in New Delhi on business and arrived a couple of days early to be able to book an organized day tour to Agra and the Taj. While I don't remember the particular company I do recall studying reviews on Trip Advisor and picking one that worked for me. I also got a second opinion from the concierge at the hotel I was staying. The tour was pleasant, efficient and well organized. While I'm not normally a tour person in this particular case I was very glad that I was part of an organized group. After observing the traffic, road conditions and chaotic parking situation in Agra I vowed that I would never attempt to do that sort of excursion on my own.
Am not sure that a train would be something I'd be willing to tackle on my own either - maybe if I was traveling with a local but never independently.
Take a look at the reviews of the various day tour companies on Trip Advisor. For the comfort, convenience, safety and security involved booking with one of them might be your best bet. Believe there are a few that do use the train to get there which might be of interest.
Might add as an aside that I was absolutely blown away by the Taj itself - it still ranks as the single most amazing thing I've ever seen.

Trains in India are an experience in and of themselves. You may be better off with a hired driver.
If you opt for the train, again I would suggest that you find a work colleague to accompany you.
You might avoid any local trains and it is probably best if you travel only in the first class section.......
Once you travel around by train there you will start to appreciate the experience but for a first time..it might be a bit much.
I'm not trying to discourage you from using the trains. I have traveled quite a bit by train in India and I love it...but just be aware, it won't be anything like taking the train from Frankfurt to Berlin.....
I wish I could offer a tour guide referral, but I was fortunate enough to travel with a local friend and work colleague when I went to Agra/Taj. A good quality hotel should be able to make a trustworthy referral. Your home travel service might also help. I'm very leery about TripAdvisor as a source for information but I know that others use it successfully.

Here's the thing...Indian's are very gracious hosts. If you make your desire to visit the Taj known to your work colleagues, I would be very surprised if they did not arrange it for you and accompany you...give it a try.

New Delhi and Agra are well connected by Trains and road.
Some people for the sake of experience go for the train journey and in Agra either go for tuk tuk or car from the station itself. They hire them from station to station basis. In order to do this you need to be street smart and should have a basic information about the distances and city.
There are some scams which are normal at New Delhi railway station (someone who would look like a railway employee but in reality a scammer would approach you and tells you that your train is cancelled for the day and there is no other train so to use your day go for private car obliviously at a higher price).
In order to avoid all the inconveniences, It is better to go for the private car and driver booked well in advance.
By using new express highway, it now takes around 3 to 4 hours to reach Agra.
Coming on to your query, there are so many companies registered on TripAdvisor, you can look for their reviews and can also search about them on the google .
In terms of costing though train would be a little cheaper but would attract more troubles as you have to do the arrangements for local travel in Delhi and Agra whereas a private car would pick you up from your hotel in Delhi in morning and drop you back in the evening.
Since you are traveling either alone or with your colleague, i think a sedan car (Toyota Etios or Suzuki Dzire) will be good for you and it will cost you around INR 6800 to 7200/- including fuel, toll tax, driver food and road tax.
Hope this helps.
Regards
Arif