The problem is that there are only so many viable paths between cities. Rails is worse than roads in this case because turns can't be as sharp and grades can't be as steep.

When there are potentially tens of thousands of property owners along that path, each property owner can derail the whole thing or add enormous costs, so they have an insanely strong negotiating position. If you were just to attempt to negotiate with each property owner, the cost would make building new rail in a populated area impossible.

There has never been any real length of railroad built through a populated area with many landowners without some kind of government intervention akin to eminent domain.

It's a tragedy of the commons situation - you might be able to buy 90% of the land for a "reasonable price" (which is hard to define - depending on the type of land use, the value of the owner's entire plot would change significantly by having a train going by every day, sometimes greater sometimes lesser), but if the remaining 10% don't play ball you can't have the train.

So the government is in a position of "we'll give you X for this, but if you don't agree we'll just take it from you", which is a... non-Texan position for government to be in, to say the least.

Imagine that you had to negotiate with a homeowner. You could find a price where you could buy their property.

Now imagine that you had to negotiate with 10,000 homeowners. At least one of them is going to insist that their place is worth one billion dollars and count on you caving because they can hold up the entire project.

That's why forced confiscation - because people try to hold the project hostage for ransom.