Tip based as per my first visit to DDN & Mussoorie visit :
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train for DDN as per timing is
i) Nanda Devi Exp (Ndls - DDN)
ii) Shatabdi Exp. (DDN - Ndls)...
more...

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(Ndls DDN) Nanda Devi Exp .
As it reaches till 6AM . and after visiting the places at DDN. till evening One can go to Mussoorie .
(DDN _ Ndls) . Shatabdi Exp.
so that Same day , Will reach at Ndls .
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For Exploring DDN ., one day is more than enough.
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Best time to visit :
(As per my visit , December end days to Mid February .)
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places visited at DDN :
→Tapkeshwar temple
→Robber's cave
→sahasradhara
→FRI DDN
→Dehradun ZOO
→Buddhist Monastery
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→ At Robber's cave,
one will have to walk slowly along the cave , with water at your foot ., As u will move ahead the level of water will increase . Its water fall . inside the caves .
ensure that u have Grip type .Footwear .
for walking inside the cave .
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→ Sahasradhara
Its again a waterfall , but here Water fall is spread with its tributaries .
At every point , Mild or drop of water will be moving . Basically the drop fall through the leaves of trees .
Here there is a place , where u can get sulphur water . Which was taken by many visitors .
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pic 1,2 - Robber's cave
pic 3 - DDN zoo
pic 4,5 - Sahasradhara