Here is a photo of the "guts" of one of these devices. It is a Bluetooth
tag - https://www.bluecats.com/product/bc313-tag/

I have heard, but can't confirm, that they are used for something far more
mundane than vehicle tracking or announcement triggering - to allow the
contractors who clean the stops and replace timetables etc to log where
they have been.

On Thursday, October 15, 2020 at 11:06:54 AM UTC+11 Mal Rowe wrote:

> On 15/10/2020 10:47, David Batho wrote:

>

> Would it be some kind of ‘transponder’ for the tram’s location, or to trigger appropriate announcements in the tram as it approaches the stop?

>

> I think you are right David.

>

> I suspect the thick white discs are new 'signposts' for the AVM (Automatic

> Vehicle Monitoring) system.

>

> I did a search of Vic tenders and they pur out a tender for an upgrade to

> the old AVM system, last year. It included the following line:

>

> *"The AVM uses radio signposts (270 on the network) and odometer to locate

> trams, which are reported as being the last-reported distance (meters) past

> last signpost, and is shown to Controllers textually, as well as on

> character-based line maps. Meanwhile there are 1700 stops on the network,

> of which the system is not aware."*

>

> So I am assuming these are the new stop based signposts, but I don't know

> why there are sometimes 1 and sometimes 2 - it could be a roll-out change

> in progress.

>

> My understanding is that the plan was to go to GPS, but it was found to be

> unreliable in the Melbourne CBD due to high rise buildings.

>

> Mal Rowe - observing change

>