I live in France. I have recently spent a lot of time studying the possibility of crossing the country without the use of TGV trains and have made a surprising and truly shocking discovery.
The French railway site, SNCF-Connect, as well as the computers in French railway stations, are rigged to avoid offering connections with ter (local) trains. A friendly booking office clerk told me that the SNCF is an enormous tentacular outfit involving many separate companies, and that the booking side is run like a profit-making travel agency. He personally had not even been aware of the bias inherent in the SNCF booking software he uses all day! At my request, he was not able to propose a trip from Valence to Lille in a day without at least one TGV, or even two!
The following question in the Forum from a year ago seems to be symptomatic of this situation, and even suggests that the problem extends to the Interrail app:
"A lot of trains seem to be blocked. When I click on a train connection in the app, there’s a red text telling me that there’s no booking option. Can somebody explain to me if this is a platform bug or if those connections may be available soon?"
But there is a simple remedy: go to the Deutsche Bahn (DB) website, which has full international coverage, to work out your itinerary. DB offers a Local Transport Only option, under Mode of transport, filtering out TGV and Intercity trains that require a reservation. Clearly, this involves a lot more work, and a longer - but in my opinion more pleasant - journey, but it saves Interrail travellers money. (I would be interested to hear of other operators who can be recommended for this task.)
I published my Valence-Lille timetable in the Questions section of this Forum, as admittedly it was not perfect and I needed help. But I got it, and I have also learnt that there are limits to what is possible. But personally, at 86 years of age, I got almost as much of a kick out of doing this research as from my sometimes difficult annual Interrail journeys across Europe!