The Thueringer Waldbahn is actually a tramway. It starts in front of the railway station in Gotha and takes about an hour to reach Tabarz. It's the line no. 4.
If you travel to Tabarz for the day then you may want to consider buying a tourist ticket for Euro 7.30 (August 2008) which gives you unlimited travel on all trams for a day and discounts for some attractions.
On the second photo is the map of line no. 4.
Tabarz is closed to A4 (Berlin, Dresden - Frankfurt) and has an own departure together with Waltershausen.
You can reach Tabarz also via train to Gotha, and then with so called electric tram Waldbahn (60 minutes).
Via airplane it´s possible to Erfurt, further with train or bus to Tabarz.
Of course, you want up to the peak, on Inselsberg. You can drive with car, there are some parking places. Or you can take the bus "Inselsbergexpress" or the little train to the high.