How cold is it ? Is it possible to go up to the top ? What is the altitude?
Train goes up Jungfraujoch - a saddle between the mountain peaks Jungfrau and Mönch.
For accessing Jungfrau mountain top you need mountain climbing experience, equipment and guidance.
Temperatures last two weeks October 2017 were in a +7°C to -15°C range - see https:/…
The Jungfrau itself is 4158m. That is a HIGH mountain. Climbing season normally ends in late September, early October so if you want to climb the Jungfrau - come earlier in the season and book a guide in advance. Also - you had better be an experienced mountain climber for this ascent.