The mountain peak is an unused area in SDV. It's original use is unknown, and CA abandaned it.
There are a few mods to get to fix it, and there is also a way to get to it without mods, hacks, or external programs.
It uses tiles from Leah's cottage, and somebody once said in the forum thread that it had a cottage on it.
If you leave it, then you will get stuck, unable to move.

To get to it, start attacking while facing south in the railroad near the exit.
Once you can't see the character and have waited a bit while attacking, start to go around the exit, and towards the northern parts of the map.
You will soon come to an unused pathway, and following that, you can go to the unused mountain peak.

Thank you for reading this guide! If you know how to get to another secret area using glitches, comment on the guide, and if you have information about the mountain peak, then please post here: http://psteamcommunity.yuanyoumao.com/app/413150/discussions/0/2425614539585364160/