Most believe that there is enough land and resources to go around for anyone that wants it.

This is completely false. It's futile to argue with this

i agree with you, I was giving you the popular view. My personal opinion on this issue falls outside of the standard ancap view. It would just confuse the topic if I went into detail on my personal view.

If I can't build a road, what stops me is the people who own land from one point to another. I can surely build an electrical power plant in my backyard, but I can't force my electrical wires through other people's property.

I completely agree and this is the key point to a voluntary system of governance. If you can't convince your neighbor to cooperate with you peacefully, then maybe it's not a good idea to be building that power plant in the first place. Government today gets around this by saying that they don't care what peoples opinions are, they're doing it anyway.

If I cross a property border by the air, isn't it still violation? Can't I put down airplanes that invade my property as well as any other invader?

Crossing through the air is generally not considered a property violation by ancaps. It's often a scenario that we discuss where someones house gets surrounded by a neighbors property, so what is the solution? the typical answer is to build a bridge through the air, use a helicopter or dig a tunnel under.

Also, it took centuries for humanity to develop flying machines, do you really think it's feasible to wait this long for something a train could do a lot better and earlier?

well it's not like this is the only idea, there could be dozens of other solutions out there to this problem, I just can't personally think of them. We rely on the free market to provide additional solutions when the need rises to a feverish level. If enough people are willing to pay buckets of money to have a solution to this problem, then inventors will devote more and more time to finding a solution.

What we don't what to happen is for government to pull out a gun, aim it across the countryside and tell everyone to get out of the way or get shot. Our goal is to reduce the violence that the state pushes onto people. If one farmer simply loves his family farm and doesn't want to move to make way for a new road, is it really justified for the government to use guns and crush his happiness?