(The railway info is valid only till June 29th, 2011).
This is the fastest train to Kathgodam from Kolkata - http://indiarailinfo.com/train/14097/7037/951. Assuming, we leave early morning on a Wednesday from Kharagpur, we should be in Kolkata station by the time the train leaves and Kathgodam by the next day (Thursday). If we take an equivalent return, this is the train to catch - http://indiarailinfo.com/train/14156?kkk=1300029542222. It leaves on Thursday. However, 6 days is not sufficient. A 7 days itinerary is minimum while an 8-9 days one is optimal.
Jeroen has a very detailed post about this route http://www.indiamike.com/india/trekking-and-mountaineering-in-india-f89/the-pindari-and-kafni-glaciers-teahouse-trek-the-details-t5894/
And so has paddy http://traveltrivials.blogspot.com/2009/03/pindari-glacier-himalayas-itenarary.html
...and here is a sample itinerary to visualize. Most information is at least 2 years old.
1a. Kathgodam - Bageshwar (160 kms/5-6 hrs). On a jeep/car/SUV. Trip should be 200-250/- per head for a shared one. LB=[Lunch Break]
1b. Bageshwar - Song (40 kms/1 hr). On a jeep. Jeroen advises to leave after noon.
1c. Song - Loharkhet (2 kms). Start of the trek to reach a KVMN trekkers hut.
Note: Certain jeeps go till Song/Loharkhet from Kathgodam. ON[=Overnight]
2a. Loharkhet - Dhakuri (11 kms). Arguably one of the most difficult parts of the trip. LB @ local dhabas.
2b. Dhakuri - Khati (8 kms). Downhill. ON
3a. Khati - Dwali (14 kms). LB.
3b. Dwali - Phurkia (5 kms) ON.
4a. Phurkia - Zero Point (7 kms). Leave early at 5.00 or even earlier. Else clouds and fog will ruin the experience. This is a must. [Early breakfast on packed food]
4b. Zero Point - Phurkia (5 kms). LB @ local dhaba
4c. Phurkia - Dwali. ON.
or 4d. Dwali - Zilla Parishad GH (5 kms). ON
and 5a. Zilla Parishad - Kafni.
5a. Dwali - Kafni (9 kms). Leave early to beat the clouds and fog. Early breakfast on packed food.
5b. Kafni - Dwali. LB/ON.
6a. Dwali - Khati. LB.
6b. Khati - Dhakuri. This is tough. ON.
7a. Dhakuri - Song. LB.
7b. Song - Bageshwar.
7c or 8. Bagheshwar - Kathgodam. Train on same day. This is one alternative - http://indiarailinfo.com/train/108/1/951/1.
Rough expense is Rs 3000 per person + 1500 overhead and train.
Notes on accommodation - Sleeping bag is a must. KVMN may not have empty rooms and will prefer bureaucrats and foreigners. http://tinkeradda.blogspot.com/2009/10/pindari-glacier-trek-short-itenary.html