We don't know, is the answer. Currently obstructionists are filing essentially bogus court cases trying to obstruct Texas Central. The obstructionists found one judge willing to ignore a century of established law in order to side with them, after losing many cases; this was overturned on appeal, but the obstructionists are hoping the Texas State Supreme Court is corrupt enough to agree to ignore a century of established law in order to pay off political allies.
The Texas Governor, Lieutenant Governor (the "we should kill grandmother to help the economy" guy), and the state AG (who is the subject of three separate criminal prosecutions) have decided to side with the obstructionists, apparently because they think this will benefit them politically (they don't show any signs of having genuinely strong opinions, since they all said positive things about Texas Central in the past). So they may be able to throw further roadblocks in the way of Texas Central. If you support Texas Central, I'd advise voting against all three of them.
I don't know how long it'll take for Texas Central to navigate this morass of anti-rail obstructionists and the politicians who favor them. Or whether they'll succeed. Nobody else knows either.
After that's dealt with -- Texas Central claims to have full funding already lined up (but has not demonstrated this to the public -- oftentimes companies can be a bit rosy about saying they have full funding).
They have picked a route and station locations. They have got most of the land they need, except for the holdouts. They have passed environmental clearance. They have picked their technological partners, so we know what rolling stock they'll use, who'll build the tracks, etc. -- the design is essentially complete and has been shown to the public.
So it's basically just (a) getting past the obstructionists and (b) funding.