How long is the Dombarton tunnel and what is its gradient?
Is this tunnel suitable for diesel locomotives and their fumes?
That's all covered in the report.
The tunnel is intended to be 4km long, with a gradient of 1 in 30.
It was originally designed exclusively for electric operation, but they believe that modern diesel locos can be used successfully. There is a suggestion that ventilation of the tunnel will be mostly natural flow due to the gradient in the tunnel, but that to help it clear more effectively, they would have a large door at the bottom (eastern) end, which would close behind west bound trains, preventing the train from drawing the exhaust up the tunnel. Once the train clears the tunnel, the door would re-open to allow the air to flow east to clear the exhaust.
If they didn't have the door, the exhaust from the bottom of the tunnel would be drawn up towards the western end with the movement of the train, and would therefore take longer to clear out of the tunnel. Apparently such doors have been used successfully in a number of other places.