What is gradient through Dombarton tunnel?
How long is it?
Is it diesel friendly?
Ideally it should be nearly level, just sloping enough to drain water from it.
Remember Cascade tunnel in USA! Built too steep and had to be replaced, and more.
If necessary move the junction up the hill to flatten the grade.
The diesels should be working at minimum power  to keep fumes as low as possible.
Diesels can work flat out on the 1 in 30 grade in the open outside the tunnel.