The train operator has recorded more than 32,690 cases of lost property since July 2022, with phones and laptops being the most common items.
Northern is now urging customers to ensure they have all their belongings before disembarking.
Leeds and Manchester Piccadilly stations are the main lost property hotspots, but items are regularly found at stations and on services across Northern’s network.
Other unusual items left behind include a cupboard, crutches, an ironing board, and a 10kg box of chicken breasts.
One customer even left their bike on the train while still wearing their helmet, and another contacted Northern to track down a plain white handkerchief.
Alex Hornby, commercial and customer director, said: “We understand that losing something can be extremely stressful so we often remind customers to make sure they have all of their personal belongings before leaving the train.
“Unattended bags on our trains and at our stations can also prompt security alerts which cause delays and disruption.
“When a passenger comes forward to report lost property our staff work hard to track down that item and reunite them – but it’s no mean feat.”
Northern, the second largest train operator in the UK, runs nearly 2,500 services a day to more than 500 stations across the North of England.
For more information about what to do if you leave something behind on a Northern train, visit their website.