I'm afraid you're probably out of luck on this one. Still, it's worth a try.

If the hotel is a reasonably good one, go back and ask about the bag at the front desk/concierge. If it was left on the curb, maybe it's still there. If your mother can remember the livery of the cab (some are distinctive), maybe they can also help contact the cab company. Given that taxis tend to be territorial, there's a good chance that the same company, if not the same driver, will be at the spot where she originally got in. Other than that, check the train station lost and found and ask if a bag was found on a train that day.

Fingers crossed. I think that the best case scenario is that she'll get the bag back minus the valuable contents, but there's no guarantee on that.

Oh, yeah, if the passport was in the bag, check with whoever was listed in it as an emergency contact, if you haven't yet. If the hotel/police have the bag and opened it to identify it, maybe they got in touch with that person. Maybe the embassy/consulate as well?

Good luck.