Pages 127 - 129 (Chapter 23 - Harecastle Tunnels) of L.T.C. Rolt's "Narrowboat" have a description of his passage behind the electric tug, and of ballasting Cressy down to give adequate clearance under the overhead power line. The description suggests a single conductor overhead and the motion achieved by a winch picking up a "second cable" laid in the bed of the canal, and paying it back out astern. He also mentions "Vivid blue sparks" from the overhead conductor.
A photograph published in Canal Boating Times, November 2015, has a fairly clear image of the tug showing a single overhead pickup.
Another photograph published in "Narrowboats at Work", Michael E. Ware, 1980, shows , according to the caption, the Chatterly end of the tunnel and one of the original Battery powered tugs, the battery bank carried in a second boat towed behind the tug.
On a slightly different note a pathe video https://www.britishpathe.com/video/electric-canals shows experiments with a two conductor overhead trolley on the Staffs & Worcester at Kidderminster.
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