Some options are not in reality "either/or" ones. Take bullhead rail and wooden sleepers as an example. Some heritage lines use them within the stations and their approaches but use concrete sleepers and CWT "out of sight".
To be fair to the sector quite a few upgrades ( admittedly mostly the cheaper and essential ones) seem widespread such as gift shops, high visibility clothing for many staff and card payments. It's the big ticket capital items particularly rolling stock, track and signalling that are the challenge.
Perhaps there will soon be a number of little used (after conversion) 769s available for diesel days services that do at least tick some of the modern rolling stock compliance boxes. But will they be as reliable as 1950/60s first generation stock
Ideally one would like all the asterixed items but as the quotes above show there are things which need updating.
I think the hardest part of running a heritage line, particularly the longer ones like the West Somerset, Severn Valley and North York Moors is ongoing maintenance of the track, signals, locos (steam and diesel)
, rollingstock and buildings. Network Rail struggles with maintenance on the national railway.
I am involved in a very short line <1 mile and maintenance is an ongoing problem. So if something requires replacement it makes sense to replace it with something more modern especialy as it may not be possible to get an original part.
Thus heritage lines will evolve, they can't be preserved in aspic.
Who knows in another 100+ years time parts of
HS2▸ might be a heritage line.