I will attempt to give a history of how this technology developed. That may help to give people an understanding as to why & how this technology was developed.
First... my background. Rapidly approaching 40 years as a radio systems specialist. For more than 20 years I've been the radio system developer for a large government agency. I have designed large radio networks for nudging 30 years.
There are a number of government agencies and public utilities that for years have complained about the need to work across multiple geographic boundaries or having to communicate with multiple other agencies. For decades vehicles and trains have been fitted with multiple radios and phones which take up space & require the user to know which one to use and when. That has often put people at unnecessary risk and cost far more than it should.
A number of people then went to bodies & suppliers asking that they develop a single "black box" that contains all of the radios & phones needed and appear to the user as a single control interface. Service providers such as Telstra had often expressed interest in doing this... I know technology vendors such as Motorola, Tait, Siemens, Simoco, and I'm sure there were others... also were keen to develop a product but were fearful of the potential barriers to jump getting "Vendor A radio" talking to "Vendor B phone" however the evolution of IP connectivity has helped to break down some of these barriers.
In parallel with all of this the railway industry were looking at carrier based standards, in particular a variant to the GSM mobile phone standard, GSM-R, which was developed to cater for rail's peculiar needs for both voice and data with security. GSM-R leant itself to the move away from rail operators providing the networks, and more of a carrier network provided technology. GSM-R still would not work everywhere, particularly where traffic is light. That is where satellite came in. The system still needed to be able to talk to the old radio systems... so over time this black box developed into what there is today.
The most important feature in all of this is that no longer does the driver or user need to decide which to use. If there is GSM-R coverage available then pressing the microphone will broadcast out via GSM-R. If that isn't available, or not working for some reason, it will choose LTE or 4G or 3G or satellite... automatically.
This approach to radio communications is not unique to rail. It is also starting to roll out in a range of emergency services in multiple states. It will eventually see a reduction in individual radio networks and an expansion of shared whole of government networks combined with 4G/LTE provided by the likes of Telstra, and satellite provided by others but controlled from a single operator console.
I hope this hasn't put anyone to sleep...