Is there a train that goes from Dover to Sissinghurst Gardens?
Not exactly. Two trains per hour run from Dover to Staplehurst, which is 5 miles from Sissinghurst. From the railway station there is a number 5 bus which goes through Sissinghurst village, but that is still over a mile from the gardens. I don't know if there are usually taxis available at Staplehurst station. You might want to google for taxi or "private hire" firms in the Stapelhurst / Hawkhurst area. Here is the 'how to get here' page from the National Trust website for Sissinghurst.