Hi Michael,
You may find this useful.
First number:
1 express passenger, mail or breakdown train en route to a job.
2 Ordinary passenger or a breakdown train not going to clear the line.
3 Parcels, empty coaching stock or Freightliner train.
4 Express freight train, fully fitted.
5 Express freight with at least a third of fitted stock connected to the locomotive.
6 Express freight with no less than 4 fitted vehicles attached to the locomotive.
7 Express freight, ballast or empty train, unfitted
8 Unfitted through freight or mineral train.
9 Mineral or stopping goods train
0 Light engine or engines with or without brake vans.
This was changed in 1968 when steam had gone to:
1 Express passenger or mail, breakdown train en route to a job or a snow plough going to work.
2 Ordinary passenger train or breakdown train not en route to a job
3 Express parcels permitted to run at 90 mph or more
4 Freightliner, parcels or express freight permitted to run at over 70 mph
5 Empty coaching stock
6 Fully fitted block working, express freight, parcels or milk train with max speed 60 mph
7 Express freight, partially fitted with max speed of 45 mph
8 Freight partially fitted max speed 45 mph
9 Unfitted freight (requires authorisation) engineers train which might be required to stop in section.
0 Light engine(s) with or without brake vans
The second character, a letter, indicated the destination of the train. The regions each had their own codes but inter-regional trains use following letters:
E Train going to Eastern Region
M " " " London Midland Region
N " " " North Eastern Region (disused after 1967)
O " " " Southern Region
S " " " Scottish Region
V " " " Western Region
The last two numbers related to the individual working (for example your first train might be 4M98 going TO Carlise, with the return service being 4S99. I think you can make them pretty much what you want without fear of contradiction.
My suggestions would be (and please, anyone feel free to correct me if I've got it wrong)
Diesel mixed freight to Carlise 4M--
Coal to Carlisle (unfitted I presume??) 9M--
Thames Clyde 1S--
Parcels from Stranraer to Carlisle 3M--
Passenger Stranraer to Carlisle (express or stopper?) 1M-- or 2M--
The last one, the oil train. Bit tricky. In current times (ie post 1968) I think I'm right in stating that this would be a class 6 train? But for your timeline, probably a class 4. So, my suggestion would be something like 4A--. Or what ever you want it to be!
You may also find this website interesting.
http://www.scot-rail.co.uk/page/Headcodes+Explained
Regards, James