I tested some Ethernet over Mains adapters at Parkend today and used the TP Link PA-411 adapters which follow the Homeplug AV500 standard.
There are 3 buildings on site (Station, Goods Shed and Signal Box) and the site is 3 phase (each building is on a different mains phase) but the Neutral wire that Homeplug uses is common to all 3 phases so this was not a problem.
Testing was carried out by accessing the web based speedtest.net service from a laptop.
When connected the laptop directly to the broadband my laptop had the full 80Mbps download and 20Mbps upload. (The FTTC broadband cabinet is less than 100 metres)
From the Goods Shed (where the telephone exchange is) I got a download speed of 25 Mbps.
From the Signal Box I got a download speed of 37 Mbps.
The adapters deliver a good throughput over the 3 phase mains to allow internet access and VoIP data.
Manufacturer refurbished PA-411’s are £11 on eBay at the current time so are very cheap.