Interrailing is great fun and despite low air plane tickets these days, there are still many reasons to engage in interrailing. For example, the fact that you can decide on the spot whether to stay in a place or leave to the next city. Once you hook up with some other interrailers, it can quickly mean that you trash your entire pre-planning and instead just decide to hang around with them instead.
Also, interrailing is something entirely different than driving around Europe with a car, even though you might have borrowed the car from a family member and are leaving cheaply in all other ways, simply because you don’t really meet your fellow travellers very much. And when you do, it is somewhere at a gas station in the middle of nowhere, with both cars heading in different directions. Meetings like that seldom last beyond just a few minutes.
Interrailers however meet mostly in trains between two transportation hubs (which usually are cities) or when they try to stay overnight in some town, preferably without having to pay for it (which usually can be done either in some park close to the train station by simply rendering the local police out of control by outnumbering them heavily in some of the main hubs or somewhere on the outskirts of cities, where no-one has thought about putting up a sign saying that you’re not allowed to sleep under someones balcony). In both cases, they have many hours on them during which they can (and usually do) engage in discussions on all kinds of topics, though at least I end up talking about mid-term (between long- and shortterm) European politics (Is it better or worse in Poland now after the end of the soviet era? Did singer with the extremely deep voice who some Swedish travellers have on a CD just not become big because he was European and not American? etc.), sites one needs to have seen that no-one knows about, or experiences with control freak border guards.
I guess taking one of the far distance buses comes the closest in that they meet one another either while travelling between or within cities (a setup which I believe also comes closest to the experience people had during the early modernization period when trains connected inner-cites with one-another, before they were largely replaced by cars that are impossible to park in inner cities and instead suburbs, gas stations and interstate/highway restaurants and motels with one another). And often I do have some of the same conversations in the bus between Oslo and Copenhagen that I would have while interrailing. However, it is generally harder to move around in a bus and although a few bus pass systems exist now, they are few and probably due to another few reasons I haven’t thought about, it all just ends up with contacts made on a bus breaking off immediately upon arrival, whereas interrail contacts often continue into finding a common place to stay, etc. .