Is Beijing S2 train Huangtudian - Badaling vice versa still operating?

Yes, it is still operated; the ticket price from Huangtudian to Badaling is about CNY7.
Nearby Summer Palace you can also visit Yuanmingyuan Ruins Park and Baiwang Mountain.

what route can you suggest we go from Badaling to summer place Yuanmingyuan Ruins Park and Baiwang Mountain?
And how long will it be?
Thank you

If you want to travel to Badaling Great Wall and Summer Palace in a day, you must set out at 05:00 and then return at 11:00. Besides, finish your lunch on the way back. From Badaling to Summer Palace requires about 3 hours without traffic jam. It is expected that you can visit Summer Palace at 14:30 and you have no time to see Yuanmingyuan Ruins Park and Baiwang Mountain. The time is very tight. So you are advised to arrange two days to visit these scenic spots.

Day one: Badaling Great Wall
Day two: Summer Palace, Yuanmingyuan Ruins Park and Baiwang Mountain

How can we reach Summer Palace, Yuanmingyuan Ruins Park and Baiwang Mountain thru subway or bus though?

Thanks