The pyjama-clad man was captured on CCTV on Monday October 6 just after 10.50pm when one of the monitors was stolen.
A onesie is a commonly used word for loose-fitting casual jumpsuits for adults.
He was accompanied by another man wearing purple leggings and carrying a spade.
Two other monitors were stolen on Sunday October 5 and Friday October 3, both in the evening.
PC Marcus Robinson said: “Three monitors were stolen over a three day period from the station. On each occasion the security surround on the monitors was forced off and the screens stolen.
“The screens, which are expensive and will now have to be replaced, are used to provide up to date travel information to passengers at the station and usually only operate in a commercial environment.
“They cannot be used as televisions but they may be able to be hooked up to a computer or a gaming station which is one possible line of enquiry we are looking into.
“First TransPennine Express, who manage the station, have now increased security measures on other screens at their stations to make them more difficult to steal.
“We are releasing their images in that hope that, despite their faces being covered, someone may recognise their distinctive dress.
“We believe they may be able to assist our enquiries into this incident and are investigating whether they are linked to the other two thefts.”
Anyone with information should contact British Transport Police on 0800 40 50 40, quoting B4 of 15/10/14.
Alternatively call Crimestoppers on 0800 555 111.