A few observations:

1. Only buy online if your origin or destination is Cancun and the other is either Merida or Palenque. Basically, you can only buy tickets online for the entire train journey along the whole line. You can’t buy online tickets for any other stations until Tren Maya gets their own online ticket system up and running. (They’re currently using a third party who normally sells tickets to concerts, etc.)

2. If you’re traveling anywhere else, you’ll need to go to a station and buy the ticket in person, unless you want to waste part of the itinerary you won’t need to travel on. (I haven’t checked, but it wouldn’t surprise me if buying the ticket at a station is cheaper anyway, because no third party is involved in issuing the ticket.)

3. Good luck finding a timetable that tells you anything other than the time a train departs from its origin station (i.e., departure time for trains starting at Cancun, Merida, or Campeche). I hope the stations have this information posted somewhere, but I can’t find it online.