Les there is a nice standard gauge railway at Lumbres between Boulogne and St Omer along the Aa valley. Picasso autorail and they're low floor access
Further south and well woth a visit is a 60cm gauge line the Cdf Haute Somme.
I can't guarantee that either are fully their wheelchair friendly but you could always ask before or let me know if and when you want to go and I will ask. The latter have a large English volunteer force.
There's not much this way.... at the moment
but we're on the flat and can accomodate wheelchairs if you're ever passing. 1 hour from Le Havre or Caen - smelly, squashy cheese and chewy cider.