The Jungfraujoch is accessible all year round by the Jungfrau railway. Possible routes are:
The train ride from Interlaken Ost to Jungfraujoch by either route takes about 2.5 hours.
The closest parking possibilities are at the railway stations of Lauterbrunnen and Grindelwald Grund.
For further maps and timetables see the Homepage of the Jungfrau Railways or the Swiss Federal Railways.