If they were taking land to build public roads, it would be legal. It is taking the land to build privately owned roads that the rest of us have to pay that Spanish company to drive on that is illegal, and immoral.
The Trans-Texas Corridor actually will be publicly-owned. It appears that space within the right-of-way will basically be leased to companies who would spend their own money, possibly with help from taxpayer funds, to put in the roadways, rail lines, and other infrastructure. The companies will be allowed to profit from the tolls and other charges while they run these infrastructure projects.
You could simply create a state agency to sell bonds (which could be sold to the Spanish company which would then sit on the board of directors), and run things that way, but why bother. The State of Texas continues to own the right of way, and regarding the remainder of the deal (the highway, operating rights, fee collection, etc.) it can once again use its power of eminent domain to SEIZE the Spanish company's property. Of course they'd have to compensate the company for that.
The Spanish company is simply an investor in a property in Texas that will yield revenue over a period of time and then revert in its ownership to the state of Texas.
This is unlike the Connecticut situation where they were taking people's houses and then SELLING the land off to a developer of commercial property.
Since your only concern is the Spanish company, we could do the same with a Texas company set up to operate a tollroad in the same place, and then they could sell stock to whoever they wished ~ maybe the Chinese Red Army.
There are many ways to do this. I recommend the Indiana technique where they use Barrett Law. The road is built and the tax payers who benefit from it pay for it. In an age of electronic marvels that could be the users!