One might assume it's forward thinking - if this were used in Japan, where there are woman-only carriages, this might be relevant.

There should never be a need for this and the fact that they exist is sad.

It's unused in reality.

Sometimes people give one-day unlimited travel tickets they no longer need to other passengers as they leave the station, which is at least against the terms of issue if not actually illegal.

I guess having a sex / gender field makes that kind of re-use slightly harder?

Which part of the National Rail Conditions of Travel is that against?

Edit: section 5.1.2

It might be needed for night trains (sleepers). Before COVID-19 in Finland you could reserve a single bed for women or men, which meant your share the compartment with a stranger of the same sex.

A model which of course assumed sex is strictly binary and having strangers of opposite sex in the same compartment is problematic, while having strangers of the same sex is unproblematic.

Same-gender compartments in trains or whatever simultaneously make sense and do not. E.g. for "cis-gender hetero woman" - as myself - these policies give me privileges such as more clean toilets but longer queues - except in tech events, where women are nowadays a minority.

In Mumbai the local trains have separate wagons for women, which was nice. There was space, women who talked to me as as "sister" singing and making handicrafts. The other sections of the train were packed as sardines in a can, and men groping each other, in particular those with fair complexion. And please don't take this comment as xenophobic or as an insult against Indian people or culture in Mumbai, this is just how it is.

At the same time, the only time I have been actually hurt by sexual harassment by a stranger happened here in Finland when a girl high on something groped me in a techno party and tried to kiss me. I got amused first, but after a while I tried to detach her. No one come to help me even I yelled that please get this girl away from me. If it had been a guy, I would have kicked him to ground, but for a girl - I could not do anything apart from just trying to push her away. Eventually we both fell down stairs and my ankle got sprained.