The RSPCA is hoping someone will remember seeing a passenger on Saturday’s 4.53pm Edinburgh to Birmingham train carrying a taped-up cardboard box.
The box, found when the train reached Birmingham at 8.51pm, contained two adult cats and four kittens.
Animal welfare officer Boris Lasserre said: “The box in which the cats were found was in a disgusting state. It was caked in urine and faeces and the smell would have been quite obvious.
“The cats may well have cried out as clearly it would have been distressing for them in the enclosed dark space – particularly as the size of the box was ridiculously small for six cats.
“The person responsible for this abandonment had clearly thought about what they were doing, it wasn’t a spur-of-the-moment act.
“It is extremely worrying to us that people are disposing of their pets like items of unwanted rubbish.”
The cats are now being cared for at the RSPCA’s Birmingham Animal Hospital and are doing well.