This assumes that flights are the option of choice for low income people to travel, but in fact low income people rarely fly with over 50% never flying and 31% flying less than once a year as opposed to high income households where only 50% never fly or fly less than once a year (https://www.mobilitaet-in-deutschland.de/archive/pdf/MiD2017_Tabellenband_Deutschland.pdf, p. 74, I’ve seen similar things for other countries, will probably be much less for the top 1%). Poor people are more likely to choose closer destinations and choosing their own car, long-distance busses (common in eastern europe) and travel less in general, not only due to the time cost and cost of transport, but also the high cost of accommodations.
Flying is one of the few areas where the distribution of flights taken is so strongly slanted by income that even a flat per flight tax would cost (by income) the 50% income percentile roughly as much as the top percentile worldwide (https://theicct.org/aviation-fft-global-feb23/ fig. 1).
If even the cost of flying can’t be touched because of concerns about disadvantaging poor people, nothing can, because flying is truly one of the things the things that is most strongly tied to income (of relevant emissions ,https://www.carbonbrief.org/richest-people-in-uk-use-more-energy-flying-than-poorest-do-overall/).
I can’t judge the state of Portugese public transport (apart from the fact that the lissabon-madrid rail link is a tragedy, mostly thanks to portugal), but flying is very obviously a big-city thing. Inhabitants of big cities fly much more because the airport with frequent and direct flights is right around the corner as opposed to rural places where the nearest airport is quite some distance away and serves few destinations.
In general poor people fly very little, which is also the case in Germany, so I can’t imagine it making much of a dent around there for the poorest. Portugal is itself mostly responsible for its transportation network and, if I’m understanding https://www.railjournal.com/in_depth/road-out-rail-in-under-new-portugese-plan/ correctly, the portugese government has chosen to invest in its road network over rail in the recent past. While just looking at the cp website it seems that prices are pretty low compared to germany or france. Similarly for hostels it seems porto and lissabon are cheaper than many less touristy cites like lyon, toulouse, cologne, genoa, … right now. I just can’t imagine it being cheaper to fly outside of portugal for vacations based on those prices. And at least for gas taxes there certainly is an alternative without large changes that is especially viable for non city-dwellers: electric cars. While still too expensive, they are much cheaper than even 5 years ago.
The last point is entirely ridiculous: The Netherlands certainly isn’t known for cheap trains and france is the opposite of a train every 10 minutes (especially outside paris), with often large multi-hour gaps between TGV connections from many cities. Most people in other european countries fly much less than people in Portugal or Spain: Portugal and Spain have one of the highest per-capita flight rates in europe (https://ourworldindata.org/grapher/air-trips-per-capita), more than double germany or france, even though spain has one of the largest HSR networks (in total and per-capita) in europe.
Flying is one of the few climate related things where the only foreseeable “solution” is a reduction. For heating, electricity, driving, steel, … there are technological solutions in the works and often already deployed that should solve the problem in the next 20-30 years. For flying there is little in the works. The aviation industry talks about SAF while missing targets for implementing them or even thoughts on how to deploy them on a large scale, while mostly ignoring the various non-CO2 related effects flying has on the climate. Flying related emissions mostly increase year-over-year (due to increasing demand) without indications of reduced emissions in the near future. And with flying mostly being for leisure, it is doesn’t need to be directly replaced.
And just to repeat myself: Flying just mostly isn’t a thing poor people do.