I've heard that the Toy Train is slow but I was thinking that the views would compensate for it.
It's a very common and obvious route yes. Let's see, I did Varanasi-Kolkata-(sidestep into rural West Bengal)-New Jalpaiguri/Siliguri-(Madarihat, for Jaldapara Park)-NJP-Darjeeling-Varanasi-(then on to Delhi).
In 1993, mind, and as part of a much longer trip. What do you need to know specifically? :)
As noted, many here touching those places in one go will have done something similar. It is indeed about (part of) the most popular route in that general north-easternly part of northern India. That's neither speaking for or against it (I really enjoyed myself); but there should be a lot of info on it, and save for some of those smaller places, it should be well-geared to tourists. If not a little overly so, by now.