There are both. They do tours by bus or you could get the train as there is a train station just outside the parks. The journey by train isn't very long.
We have done it a few times - there is info about it in my trip report from Easter 08 (we did 2 separate day trips then)
It takes about 30/40 minutes and the trains run every few minutes. Im not sure if they continue all through the night but run quite late in both directions