NRN was indeed designed for national, railway based coverage, alongside a overlapping emergency radio network. It was in Band 3 around 200Mhz
The NRN was a trunked radio system with each basestation allowing a single conversation at any time, the overlapping emergency system 'listened' for emergency calls and immediately cancelled any ongoing conversations when an emergency call was heard.
From a NRN handset you could dial any BR extension phone, and there were specific dialling codes for ECR's etc if I recall correctly. As the network used telephone extensions and standard PABX routing there were rules against Drivers using the network for calls that could (or should) be made from an SPT.
The RETB systems used the same basestations with a different control set up.
CSR was developed primarily for DOO, and was designed to be secure and also to route the call to the appropriate signaller for the area, hence it could be used by drivers instead of an SPT. I dont recall the CSR system allowed any calls to the BR telephone system unless the signaller re-routed the call.
All CSR radio conversations were recorded, it was only in the last few years of NRN that those conversations were also recorded.
I travelled on many units in the NSE testing CSR and I cant remember seeing any dual equipped NRN / CSR units.
I'm going back 20 odd years now, so happy to be corrected by those with better memories.....
