In line with the period chosen locomotives and stock coincide with what was running on the Waverley route at the time. Type 4 diesel locomotives were mainly in charge of the main line expresses, whilst smaller type 1 & 2 loco's handled freight, local passenger and branch trains.
Common types included what were to become class 17/24/25/26. also Class 20's were documented on the Langholm Branch and Newcastleton local services. For the purposes of the layout Carlisle Kingmoor & Edinburgh Haymarket allocated locomotives were the obvious choices, though other Scottish depots and possibly even loco's from the North-East might get a look in.
Seen here with work still under progress on both layout and loco! The '25 was the first loco to appear- it has been renumbered as an example known to be based at Carlisle Kingmoor in the required time-period. Other alterations include modyfying the roof & underframe as the model was a 'boiler fitted' example- not relevant for the time or area. This loco has received a reasonably heavy-weathering and is still awaiting new wire handrails in place of the Bachmann plastic ones.
This locomotive is the Bachmann D8307 version- it is currently awaiting re-numbering to D8324 which will feature 3 piece mini-snowploughs, and slow speed control, which requires some small modifications to the cab fronts- this loco was known to be allocated to Edinburgh Haymarket, and would have been delivered already in rail blue livery. This loco currently carries Bachmann's own factory weathering.
This loco is really a little large for such a branch-line setting- though they were commonplace on the Waverley route along with similarly sized 'Peaks'- a final identity for this loco is still being sought!
A recent aquisition is a Class 24 in 2-tone Green livery with small yellow end panels. Likely to be renumbered as a Haymarket loco & placed in charge of the passenger train till I can scrape together the readies for a '26!
The loco has now been weathered- after much help and advice from the RMweb forum:

A further aquisition is Blue Class 25/3 D7671- this has already been weathered by its previous owner, and is nicely lightly weathered, just the thing for an almost new loco- these loco's really are a gift to the EM modeller as re-gauging is almost childsplay- just pushout the existing wheels and adjust the pick-ups, a 10-15 min job once you have done a few.

A well-detailed class 27 has joined the Deadwater fleet, its a Lima body with Ultrascale wheeled chassis, it requires a little backdating as it is seen here in mid 1970's condition.
Certain loco's have been 'earmarked' for a quick departure to the West Highland line, having come into the ownership of 3 very nicely detailed/weathered Heljan Class 27's
There will be no more type 4's till I finish the Castle Street project, unless of course they get really desperate to shuffle those no 'nose seam' ones off really cheap!
Unlike the steam layout with the token green diesel- you've already guessed- there maybe the possibility of the odd steam loco substitution- I'm hoping when Heljan do finally bring out the class 17 it will be with a dummy chassis so it can be rescued with your loco of choice!