John Torrance, in his letter to PE August 2012, Diesel's here to stay, got one detail wrong. He states that there are only four trains a day from Edinburgh to Aberdeen, making electrification of this stretch of railway line uneconomic.
From my vantage point in Dundee I can assure him that there are way more than four trains a day. The National Rail online timetable lists 36 trains from/to Edinburgh. With other services, e.g. from Glasgow, the total number of trains per day between Dundee and Aberdeen rises to 60.
The question remains though at which stage electrification would be considered economical.
Joachim Neff, Dundee