If you're a responsible freight truck company, you want to avoid this situation at all costs. There's always cargo that wants to be shipped - the only scarce resources are drivers and tractor units, and some companies even let multiple drivers share the tractors to maximize the usage of their tractors (that really depends on the company though, there are also companies where every driver "owns" his tractor).
> The fact remains we don't have enough rail to service most of the country, and nobody is paying for new rail
Yes, indeed. The beginnings of the stock markets actually were railroad construction companies, but these days the financial markets only care about the next big unicorn and not about creating something with real long term value, which a train track certainly is given that, once the track is laid, the land belongs to the company and the only major investments required are 50-year overhauls and regular maintenance. It's a shame.
Trains have several single points of failure for a larger load. When there is a problem, all of the load is affected, it takes longer to repair, and problems with both the rails and the trains happen regularly. And trains have more complex maneuvering than trucks; switching lines is a non-trivial time-consuming matter and operators have a tough job just on the existing lines.
The fact remains we don't have enough rail to service most of the country, and nobody is paying for new rail, not to mention the issue of fitting more track on the existing land and bridges used for rail. Demand has been increasing steadily on rail, and only so much traffic can possibly run on the rails - which are shared with passenger trains, btw. To switch to trains you'd be adding 230% more traffic to the rails.