A large theme of this alternate fictional universe is to give things reasons to exist. I've changed a lot from the Reverend's version, as I wanted to backdate the opening of the main Sudrian railways and give the island a chance to industrialise rapidly at the beginning of the 20th Century.
A big part of any railway is the traffic demands, these will guide what kind and amount of stock the line should have on its books.
For 'fun', I've begun to devise timetables for the NWR main line and the branches. This will tell me how many trains need to be run per day, and thus how many locomotives, coaching sets etc. are required.
The period I have chosen is high summer in the mid-late 1950s. Steam is still king (and the NWR remains un-nationalised here due to some complicated legal gubbins I won't go in to here. The IoMR remained privately owned long after 1947 so there!).
I so far have been working on these services:
The Peel Godred, Kirk Ronan and Norramby branch lines still need to be worked out.
For each service I've worked out the distance, average speed over that distance, and dwell time in stations to give me the overall travel time.
The complicated bit is trying to work out how to connect services. For example, can people from town A get to town C in a decent time if they have to change trains at junction B?
The other thing that this tells me is the type of loco required, and how many of them.
Main Line (Express) - class 5-8
4 train sets
Main Line (Stopping) - class 3-5
9 train sets
Brendam Branch - class 1-3
2 train sets
Ffarquhar Branch - class 1-3
2 train sets (1 passenger, 1 quarry workers) + railcar
We can assume that each 'set' requires a loco. Plus a spare should be kept on the books for each group, assuming something will be undergoing maintenance.
These locos are solely passenger or mixed traffic, goods locos are another matter. I'll need to take a look into the industries we need to serve and work out traffic requirements on that front....