You can see timetables ("fiches horaires") online from the SNCF website for the Calais-Lille services they run on:
https://ter-fiches-horaires.sncf.fr/publish/FH_Lille_Calais_horaires_théorique_du_14_decembre_2025_au_3_juillet_2026.pdf (route timetable)
https://www.ter.sncf.com/hauts-de-france/se-deplacer/fiches-horaires (Hauts de France, the region website)
On the timetables above they are the ones in pink which run the full route to Calais, line K71.
If you're sticking to the Lille's day ticket, the Ilevia, then the boundary goes as far as Armentieres, the first stop on the route. If you're a bit more adventurous then you could travel halfway along the route to Hazebrouck, located near two junctions and a small yard - and is apparently great if you like a bit of freight spotting - and catch a returning loco-hauled service back to Lille.
Elsewhere, there are posts on this forum where loco-hauled stock run around France, though Lille-Calais is the only one in that area AFAIK.