And 1 in 5 might be nominally disabled, but are they disabled to the point of needing assistance to board a train?
If you look at the data I have linked to, you will note a breakdown of disabilities by category which should enable you to calculate that.
Quoting directly:
Among disabled working-age adults, the impairments reported most frequently were mental health and mobility (in both cases, by 42% of this group).
For those disabled and over State Pension age, 63% reported mobility as one of their impairments. Stamina (or breathing or fatigue) were reported by 38% and only nine per cent of this group reported a mental health impairment.
Because it would cost astronomical quantities of money.
Providing staff on every train would end up costing insane sums of money.
This is referring to one solution when you're asserting this, rather than any of the others proposed by
@Bletchleyite or indeed alternatives that weren't considered in his post.
This also fails to take into account costs that would fall on other services as a result of failing to provide reasonable adjustments across the network - for example, a person losing their independence and requiring increased levels of care and support.
And most importantly, I feel you fail to acknowledge the deep impact on an individuals' dignity, livelihood and ability to live a full and independent life. These all come with monetary as well as societal costs, and a good analogy is the significant reduction in preventive public services that then increase pressure on other organisations, for example the NHS. David Haslam's book explores this in more detail:
https://www.waterstones.com/book/side-effects/david-haslam/9781786495365
In any case, there is a legal duty for the railway to provide reasonable adjustments and that, I hope, is not in dispute. The question should be what is the most efficient and appropriate way of providing that reasonable adjustment for the passenger, in a way that is not demeaning or removes their ability to access services as anyone else would.
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As an aside, you also might want to reflect on your use of "insane" as a pejorative in a discussion about disabilities.