The theft happened just before 10.30pm on Tuesday, June 24.
British Transport Police have now released CCTV images of a couple they want to trace.
PC Mark Dove said: “A man and a woman entered the outlet and ordered food at the counter. The woman then picked up the hand-held card machine and passed it to the man she was with who placed it inside his jacket pocket.
“The couple then collected their food and left the shop with the machine.
“We would like to speak with the woman and man, pictured, as we believe they may have information which will assist with our enquiries into this theft.”
Anyone with information should contact British Transport Police on 0800 405040, or text 61016, quoting reference PSUB/B3 of 8/7/2015.
Information can also be passed anonymously via Crimestoppers on 0800 555111.