Separately, this is the other one, actually found just east of Craigentinny. It appears to be made up of MkIs with the windows plated over, and I reckon it was either a test train or the 1980s version of the HOOP. It should be visible from the A199, and certainly from the ECML.
http://maps.google.co.uk/?ll=55.947175,-3.105099&spn=0.000871,0.002642&t=h&z=19
You are pretty much correct: The stored rake of Olive Green former mark 1 vehicles in Portobello sidings is the former ECML Overhead Line Maintenance Train, converted from mark 1 passenger stock at the time of the ECML electrification AFAIK. They may also have well been deployed during wiring activities, and certainly when the wires first went "live". The vehicles stored at Portobello comprise:
975681 - OHLM Pantograph coach
975682 - OHLM Pantograph coach
975685 - OHLM Generator & Stores coach
975686 - OHLM Stores coach
975687 - OHLM Stores and roof access coach
975688 - OHLM Office & staff coach
If you scroll down, this page provides a number of details of the individual Overhead Line Maintenance vehicles, by clicking on the relevant links:
http://www.departmentals.com/photos.php?cat=975
Here's a couple of the Portobello vehicles:
http://www.departmentals.com/photo/975682
http://www.departmentals.com/photo/975686-1
As you will notice, there used to be many, many more converted vehicles dedicated to this task at one time in the past. This post from that website details the in-service formation of the former Portobello OHL Maintenance train that remains stored there to this day:
...back in the day each OHLM depot ran a six car formation (plus one or two cable drum wagons)
The scottish set allocated to Portobello had
975 681 pantograph coach
975 685 stores and generator
975 686 stores
975 687 stores and roof access
975 688 staff office
975 920 cable drum carrier