It is no secret that I think MQTT has great applications in Model Railroading. With the release of inexpensive ESP8266 (and soon ESP32) WiFi MCUs and now the Raspberry Pi Zero W, I think we're close to reaching a tipping point which enables the wide deployment of affordable hardware.
Hackaday ran a nice series of articles on MQTT last summer. Excellent stuff.
A video tutorial for creating a door-bell with push notifications is another neat tutorial video:
The real challenge here will be making this accessible to "normal" hobbyists.