Freight trains have a much lower fraction of human labor as cost vs. benefits relative to trucking or cab driving. 1 truck and driver carries 20 tonnes. 2 engineers on a freight train carry close to 20,000 tonnes, so there is only 1/50th (ish) the benefit.

No. One form of transportation doesn't have to exist for everything. Rail is the long haul and trucking is the last mile.

Autonomous trucking should start out with its own dedicated part of the highway between distribution centers. We are trying to build autonomous driving to accommodate our current road structure which humans currently drive on. A dedicated / isolated lane with sensors, etc should be the start. Start small and then you can expand after getting feedback data. Everything is always so costly and unattainable because everyone wants to implement it with the current infrastructure but its not needed.

For reference we already do these types of dedicated lanes with ezpass express or HOV lanes.

Caught that did you? :-) The one interesting disruptive thing might be paved roads vs rail infrastructure. It is easier to have a steerable vehicle turn on different paths than doing railroad switches. So the underlying physical plant of building things might allow for rapid deployment vs rails.

we do have semi-autonomous trains, in the US at least. i'm not sure about elsewhere. PTC stops a large class of disastrous operator errors (overspeed, missed/ignored signal, etc.). it is almost entirely complete on all of the class 1 railroads in the US by now.