I'm having my interrail years now, about 55 years after everybody else.
I do 2 short continental train trips a year, spring and autumn, aiming to increase my footprint to as many European countries as possible, sampling their trains and general touristy, cultural things.
I like my routes to connect up into a continuous network to/from the UK, even if rather indirect.
Starting from Manchester (sometimes Liverpool) I fly to where I left off the time before, do a 3-4-day tour usually across national borders, and fly back from the destination. Day trains only, overnight hotels near stations.
Eurostar is out of the question, as it doesn't get you far enough on day 1 to be useful, and the hotels are the most expensive bit of the trip.
When living in the Thames Valley I used Eurostar/TGV a number of times on family holidays (eg Annecy, Bordeaux, St Raphael) as well as Paris and Brussels, so I know how it works.
In the past I have also used ferries (Harwich-Hoek, Le Havre-Portsmouth) and the Dover routes, but they are just too slow these days.
It's also incredibly cheap to book an LCC 6-9 months ahead - eg Manchester-Munich and back from Sofia for £60, going Munich-Zagreb-Belgrade-Sofia by train between the flights.
I've had a couple of flight cancellations in 12 trips (BMI, Flybe) which luckily did not affect the rail part of the trip, otherwise no logistics problems.
So far I can trace several long rail routes across Europe eg Malaga-Paris-Hamburg-Tallinn (OK includes 2 ferries!), and Wick-Strasbourg-Milan-Naples-Siracusa, just not all in one go.
I'm hoping to reach the Aegean, Black Sea and Africa (Morocco) before I'm done.
There's too much culture and scenery in Europe to spend it in the northern Eurostar flatlands, or travelling overnight (you don't see much of a country in a sleeper).
At the overnights I usually get a 24-hour transport ticket and explore the local metro system, while seeing some of the sights.
I also once had an unplanned visit to a Budapest military hospital, but best not go into that!
I also try and visit the opera or a concert on my trips, and so far I've managed Mozart in Berlin, Verdi in Naples, Wagner in Estonia and Dvořak in Hungary, all excellent experiences.
I can also tell you how user-friendly foreign rail web sites and ticketing systems are, and what the best deals for seniors are (or were when I passed through)!
I use "Saver" fares locally, buying e-tickets well in advance from the national TOC.
It's never worth buying passes of any description, they are poor value these days.
Nor do I use the UK-based ticketing sites, where you frequently can't get the best fare.
Part of the fun is fathoming the best deal on an inadequate English translation of a local web site.
I just hope the ease with which we can cross borders today is not lost with all the security and immigration fandango that is growing every year.
Not to mention leaving the EU and its citizen travel benefits.