This morning on the Dublin to Belfast Enterprise, one person sitting opposite me and another beside me began applying their makeup.

At the same time, they were drinking Red Berry, Pineapple and Passion Fruit West Coast Cooler.

Perhaps, to them, it was simply part of the journey: getting ready, having a drink, and making use of the time. I doubt either intended to cause discomfort. But the combination of cosmetics and sweet, fruity alcohol filled the small space around our table.

The train was crowded. There was nowhere else to sit and no possibility of moving away. I simply had to remain there, with the smells coming from beside me and across from me, until the journey ended.

For an autistic person, scent is not always something that can be politely ignored. A smell that seems faint or pleasant to one person can become overwhelming to someone else. Others sharing the carriage may have asthma, migraines, nausea, or health conditions that make fragrances and strong smells difficult to tolerate. Most of this is invisible.

Accessibility is often understood in terms of ramps, lifts, accessible toilets, and reserved seating. All of those matter. But accessibility is also about the atmosphere we create around one another. It is about remembering that public transport is shared space—and that the air is shared too.

This is not really about makeup, or even about a particular drink. It is about the assumption that whatever we choose to do in public affects only ourselves.

It does not.

On a crowded train, other people cannot always move away. Courtesy therefore means more than keeping our bags off seats or our music inside our headphones. Sometimes it means considering whether the scents we introduce into a confined space may be difficult for someone else to escape.

The air on a train belongs to everyone.

Sometimes kindness begins by remembering who else has to breathe it.