The OP's user name suggests that senior tickets might be of interest?
Every country/operator has a different take on senior fares, from no discount at all to nothing (ie free!).
There is often a railcard-type scheme like we have, but discounts vary and sometimes are not worth the annual cost of the card for a fleeting visit.
Sometimes the discount is only on full fares and not the already-discounted ones.
The starting age for "seniors" varies between 60 and 70, usually linked to the pension age in that country.
France is confusing with a poor-value national scheme, but sometimes very good local schemes (on TER services).
SNCF has on-demand senior fares on some TGV routes, not on others.
The best country scheme I've found is Spain (Tarjeta Dorada, €6) which gives 25-40% off normal fares (but not the cheapest advances).
The catch is you have to buy the card at a Renfe station, and you need the card number to book discounted tickets.
Belgium has a flat fare of €7.20 (return) for seniors you can buy on demand, including some cross-border ones (not to Luxembourg/France).
Norway, Finland and Portugal (long distance) have half-price for seniors on demand (75% discount in Czechia on 2nd class fares)
I thought the schemes in Italy, Switzerland and the Netherlands were useless for casual tourists like me, and Germany/Austria only marginal.
No senior discount on Eurostar (or airlines/ferries for that matter).
Some countries in Eastern Europe have free fares (2nd class) for seniors, you just pay the supplements for express/1st/sleeper etc.
Beware that some of these deals (eg in Hungary) are only available to EU citizens, which we are not any more.
Some capital cities have free tickets for seniors for local travel available to visitors (eg Prague), but most (like London!) have no discounts at all.
This site gives a country-by-country indication of what's available (though it's written for Americans):
Senior Train Travel Discounts in Europe (tripsavvy.com)