(Apols if this has been answered before; can't do search on the mobile interface) The green emergency signs in the loops that were put in a few years ago point to the nearest emergency exit. Between Richmond and Parliament on the Caulfield Loop there appears to be one midway (I've never been on a train going slow enough to see it though). Are these just cross tunnels to the other loops, or are they stairs to the surface? If so, where do they come out? Are there others between the other underground stations?
I believe there are only two 'real' mid-tunnel emergency exits, one in the Caulfield Loop between Parliament and Richmond, the other in the Northern Loop between North Melbourne and Flagstaff. Here is the emergency exit signage showing the distance to the latter exit, presumably all other 'real' emergency exits will be marked on the mid-tunnel signage in the same way.[img]http://railgallery.wongm.com/cache/city-loop/E115_0743_595.jpg[/img]
The Northern Loop exit can be seen from slowly passing trains, with a brightly lit set of concrete stairs, not a ladder or other 'maintenance only' type access. The cross passages are much harder to spot, as there isn't as many lights in them. I'm guessing the higher level of lighting at the exit stairs is to meet evacuation route requirements.
Last year there was a news article in The Age
about safety issues in the City Loop, including supposedly locked emergency exits:http://www.theage.com.au/victoria/city-loop-safety-fears-20110921-1klbd.html
I am theorising that the 'locked exits' mentioned in the Age
article are the ventilation shafts and cross passages, as these have a reason to be closed to pedestrian access (not designed to meet evacuation standards) while the proper exit stairs have one way doors and alarm systems to prevent unauthorised access, a standard feature of fire escapes anywhere else in Australia.