More passengers does bring down costs to a point, should you be filling empty seats. However, the railway has been a victim of it's own success, as at some stage you need to run more trains, therefore cancelling out some of the previous efficiency gains in terms of train crew and train set utilisation.
However, if lines are better utilised, that will bring efficiencies in terms of overall unit maintenance costs, utilisation of non-train crew staff such as those on stations and signallers.
I dread to think of the cost of some of the lines where mechanical signalling still exists, with signallers doing nothing for 50 mins out of an hour with a box every few miles. Abolishing this sort of equipment should have been an early priority for the privatised railway as should have been developing low cost signalling systems for secondary and rural line - ironically something BR did quite well. This is only just starting to happen.
Something the railway press, especially modern railways, have got entirely wrong is blaming things like speed for inefficiency. The truth is it's the slow trains on branch lines trundling up and down are the most energy (per passenger) and are the most cost inefficient. Not the fast trains, where you get better stock and crew utilisation, and shorter journey times attract people out of cars, coaches and planes onto the railway meaning you get a better load factor and make more money. The East and West coat main lines are the biggest earner per passenger the railway and some of the biggest revenue earners in the industry.
Yet all to often the private railway has sought to increase padding in timetables, which extends journey times, to avoid penalties which don't exist for competing forms of transport. So you get the Government spending millions on improving the A46 between Leicester and Lincoln, the the rail service has extremely padded times (including sits of 20 mins or more at Nottingham) which surely reduces the attractiveness of rail, and decreases stock and crew utilisation.
The biggest cost in rail is not fuel, or track, or the trains themselves, it tends to be staff.
For those that worked on the railway from the transition between BR and the private railway, there was a massive increase in middle management.
Most other businesses have tried to reduce management count, often they are an expensive luxury. Always in a business, one of the key things is to ask 'what does this role bring to my organisation in terms of generating extra revenue?'. Many of these managers are on fact finding missions or doing admin work. Other staff could easily be doing these roles, or telling you if there was correct communication. Instead the railway often proceeds down a 19th century them v us culture, that costs money in the long run.
With such high wages being paid in the industry, in theory there should be no problems attracting the brightest and the best. So you would think there would be no issues with people having more skills. In fact the opposite often happens, either because the industry has been broken up, or managers simply do not trust people who are often 40-70k per year with overtime to do a good job. When things go wrong, the solution is always to bring more managers in, when for the wage levels the railway are paying the staff, they should be telling you managers what is going wrong and writing the reports themselves, and other managers should be listening.
Unlike most other business the railway does not seem to want to multi-skill staff, or professionalise roles (see above). So you have perverse situations where there was a points failure, and previously station staff could wind points, now the railway was employing extra people to do this role. Conversely, when stations were short of dispatch staff, and trains were leaving late, the incident staff were sat with their legs up reading the paper! Meanwhile the driver is sat in the cab, where quite honestly there is no reason he cannot work under signallers instructions to wind and clamp points to get things moving.
Opportunities have been entirely missed. Why are separate ticket office staff still employed, when shops on stations can sell tickets, or outside industry can offer these services? There are many other roles that have a questionable existence, or could be done by other parties at lower costs.
The list goes on and on, it's telling the railway of 1900 that run on paper, steam, oodles of staff and far more complexity, as well as requiring more maintenance made profit and the so-called super streamlined plastic railway of today despite far less staff and far more technology barely covers its costs.