You could take a train from Delhi to Kathgodam. Shatabdhi express. Then on hire a cab and head towards Nainital. Another express is Ranikhet express which starts from old Delhi. Coming to bus mode of transport, it takes 7 hours to reach Nainital from Delhi. There are daily bus services available. You can check with Red bus, Yatra and Goibibo. Kindly check once more regarding travel restrictions before planning and booking.