I suppose in hindsight you probably shouldn't have let them spot Luc doing that, but what's done is done.
The adventure still works if the party doesn't get off in Sid Minos, but you lose some really important groundwork for later revelations if they never go to the isle of Odiem and the Crypta. Like, if you want to let the party just corner Luc and arrest him, Ottavia might be persuaded to tell the party that they're meeting people in Vendricce, though her geas would keep her from saying who. Or if the party outright kills the two of them, they could still, I guess, watch to see what the train does when it gets to the end of the line, and go up to talk to Lya Jierre when they spot her.
But no, that's not as fun. Don't let that happen.
Instead, try this:
Luc's first gambit failed, so he tries a different approach to shake the party's pursuit. Ottavia stays at his side at all times, fearful of him being abducted. When the train is pulling in to Sid Minos (or if the party comes ANYWHERE NEAR HIM BEFORE THEN), he immediately triggers a handheld device and both he and Ottavia blink out, and the air is suffused with the scent of burnt engine oil. (This is basically the Gatecrasher Charm effect that a few Ob folks get, including Lya and her allies get in adventure 6.)
Luc and Ottavia end up tumbling into the Bleak Gate and get hurt from whatever speed the train was traveling when they land in the grim parallel reality. But then they hustle for a contact Ottavia has in the city. In a few minutes they pop back to the waking world, and get hurt some more, but then they get help from a healer. Ottavia gives Luc her Ob ring to protect him from divination, and then she tells him to lay low and get back on the train before it leaves tomorrow; she's going to stay and try to lure the PCs off.
When the party reaches the city, concoct a way for them to get a lead to where Ottavia went. Maybe if Bree is still on the train, she'll go in search of them. Maybe they go to a local priest and get them to cast Locate Creature, which points the way to Ottavia. If they divine for Luc, the ring protects him and instead makes the divination act as if he's with Ottavia. Ottavia is, just then, heading to Odiem.
Ideally your party pursues and gets stranded for the night on the cursed island.
Oh, and a random fun idea.
It is generally a bad idea to mix different types of dimensional magic. So the next time the party tries to reach into their bag of holding, they discover that no living thing can enter it. They can turn it over and dump stuff out, or fish things out with like a hooked cane or something, but no living matter can enter. If they had food or water in there, it comes out eerily denuded of nutritious essence.
How does that sound? You really really ought to have plans and contingencies to get the party to Odiem.