Shanghai, Nanjing trip

Hello,
I'd like to inquire about several things about my trip to Shanghai. Many thanks.
1, When traveling from Shanghai to Nanjing, do I need to buy train tickets in advanced on the internet, or can I buy it at train station, 30 minutes before departure?
2, If I´m not mistaken, the best way how to get to Niushousan forest park in Nanjing is taking a taxi, would it be a problem to find some taxi to get back? When I arrive, should I ask a driver to get back in 6 hours for example? Would you give me some advice for this trip?
3, Now, when Google maps doesn't show Chinese metro stations anymore, could you recommend me some offline map of Shanghai that shows metro stations?
I´m very thankful for your support.
Best regards
Ladislav