Your looking at it the wrong way. The rail service should not be a purely profit and loss case (remember that thinking in the 60s). It should be a tool for improvident of the rural area. Like all seasonal business, you sell hard in the summer to make up for losses in the winter, and both those lines are very busy in the summer!
With the WHL your main attraction is free, i.e. the outstanding scenery, however no one is going to use your service if its clapped out crap. These thing need marketed properly too. Scenic trains were popular in the 80s/early 90s only to come to and end due to privatisation.
With your argument a certain western ferry operator would be running about with ships from the 50s with no amenities.
Can I make a comparison?
The traditional ferry service to orkney is provided by a full service ferry which operates twice a day, three times in summer between Scrabster and stromness. By full set I mean that it has a large passenger capacity, and offers everything from a cup of coffee to a full meal. It is highly subsidised, and because the boat weighs thousands of tons, it has a huge carbon footprint.
The recent competition runs a bare bones service between Gill Bay and St Margaret's Hope. It operates up to five times a day, uses much lighter boats which use a fraction of the fuel, with a much smaller crew. It needs no subsidy, and carries the vast majority of the traffic. They're getting another new boat.
It supports the economy of Orkney by allowing people and goods to be carried at lower cost. And personally I think the AYR service to Mull should be similarly provided by a smaller, more basic boat operating a much longer day. No
These tourist-orientated luxury services make no sense unless there are enough people prepared to pay fares which are a multiple of the normal fare. In some cases there are: you can travel between FW and Mallaig and back for around £14.40 on a 156 or £35 on the unsubsidised Jacobite.
So, if you think a luxury service on the rest of the WHL is justified, you have to ask whether there is a market prepared to spend a multiple of standard fare for your enhanced service.