This will not help you. The new style mobile pass-with the QR-code IS the ticket-though there are indeed a few isolated posts that in some remote corners the railways staff has not been properly instructed-or maybe were but did not pay attention- and put on pressure on unsuspecting and often easy to frighten (putting pressure to claim ´fees´ without receipt is not a totally unheard thing in these areas) young travellers. The best thing is likely a local-in their language-statement from the ticket counter-if they are willing to understand it or even able to.

If it really cannot be solved-pay up, get receipt, and claim back afterwards-yes, I know, a not very satisfactory but sometimes unavoidable way to handle things

Thank-you so much for your advice. 

We winged it and they let us on without any reservation. There is a second class seater which has no reservations that anyone can go on. Despite what the ticket person said. 

For anyone in the future get on a few stops earlier than the main station in Montenegro. The ticket police are really nice and they know about barcodes even though they have only paper. 

Bring enough food, tissue papers, hand sanitizer, water to last you the 12 hours. - We arrived at 8am instead of 6am.   

Please note that everyone will sit on the platforms without a care in the world. Go down to the right hand side of the train station one before the first class sleepers. 

No reservations are required and your ticket WILL WORK. Croatian is the language to google translate. 

The train is a bit mad and once you reach the check point it starts to get cold. Around 1am. So make sure you bring something to change into. The people on the train also get quieter. Once you pass the check point I would recomme

nd that you sleep for the remainder of the journey. 

It's an adventure! Have fun

We hope this advice can help someone else:) 

Cormac and Sarah x