Hello Dave.  I can't say a lot in detail but I've worked before on desktop client software which monitors the telemetry served up by microcontrollers embedded in communication equipment used by French Metro systems.  It all stems from an overhaul of comms equipment that happened after the Mont Blanc tunnel fire highlighted that the systems used by all three emergency services involved could not communicate with each other.

The original protocol we developed was bespoke, but since the explosion in IOT and other such things customers are now much more fussy about the protocols in use and having them meet standards.

A lot of folks are trying to stretch the point with SNMP, especially version 3 since it supports encryption, but in my opinion this is an incorrect use of the protocol which is designed to do exactly what it says on the tin, manage networks.

Since MQTT already has found use in railway systems and complies with Cenelec we may be able to pursuade customers to abandon their misuse of SNMP and adopt MQTT