Different country, different operators but same problem. Make the trains go where people want to go at times they want to go and with adequate capacity for passengers and their luggage at those times and do all that at affordable prices for the target clients - seems simple and logical but it's far from that.
I am in the throes of preparation for a simple extended w-e cycle-camping the Brest-Nantes canal with my daughter next may (a known tourist route, nothing bizarre or exceptional). The train link necessary to go from end to beginning is a sheer nightmare and is determining the direction we go. Looking at bike travel guidance from the local regions I get "Don't travel with a bike during our peak times and you won't necessarily have space but you can't reserve it outside that", "Bagged folders accepted at all times", "Tandems, trailers, cargobikes and anything else abnormally big refused" (which almost certainly excludes non-folding 'bents, and possibly some folding ones!) from the Brittany region, "Avoid peak times, travel after 20h on holiday week-ends" for the Loire-Atlantique and, if you want to tour the Loire Valley, "33 bike places per train - rising to 83 places in high season". I am wrestling with the problem of a maximum of two practicable train times for the return link and the probability that the folder is going cycle-camping again. I don't normally use trains and this experience won't encourage me!
Could do with a survey but then the SNCF would not pay any attention to outlying regions, just the parisiens; going anywhere from Limoges already involves a three hour trip to Paris to change lines, stations etc!
Good luck with your Sustrans survey