Human drivers still do meaningfully cost money to employ, and they do annoying things like needing to sleep or take rest room breaks. Autonomous trucking would completely revolutionize the industry.
That said, it's not at all clear to me that developing autonomous trucks for long interstate routes would be meaningfully cheaper than laying rail on literally every high-traffic route on the planet, which uses perfectly-understood technology and has entirely predictable returns.
Autonomous driving in good conditions on highways is already working quite well, but once it's wet/foggy and there are animals wandering onto the roads and other cars are behaving unpredictably... those are not straightforward problems to solve.
Also, for the unfamiliar, 'guard' here does not mean an armed guard.