Oberland does really need walls or just some defenses? Also is there concreete picket fence to give concrete. i seen ppeople refering using it. and how far up minutemen questline before i do rr or the main questline?

If you intend to do settlements, you need to balance defense with food water, usually 2:1. For example, if your food and water are both 5, you should plan on 20 defense. You don't need walls, but they're helpful, as they'll protect settlers from being shot at.

Minutemen quests don't go to the Railroad. Instead, as you explore downtown Boston, there's a tour guide that will fill you in on the history. When its done, you'll get the quest to "follow the Railroad". If you want to know where the entrance is, say the word. Otherwise, you'll have fun finding it.

The Main Quest will eventually take you to the person who shot your wife/husband. You'll need to do this quest in order to bring in the BoS. Don't worry, you won't miss it. It's actually a pretty awesome cutscene.

Once the BoS has arrived, the MQ ending is entirely up to you.

Oberland Station could use a walled South side. I used a Minefield on both ends of My Wall, but the Settlers loved to run through the Mines to attack the Enemies.

I cheat and use Steel Prefab to make a wall for Oberland Station.