The train operator announced today that some seats would be removed from its months-old Azuma trains to accommodate more bags, following complaints from passengers about lack of room.
The Rail Maritime and Transport union (RMT) warned last month of impending problems as trains got busier over the festive season.
General secretary Mick Cash said: “The available luggage storage has been found to be woefully inadequate.
“With the upcoming Christmas period, there will likely be a large increase in health and safety issues and stress for frontline staff.”
LNER said its "world class" Azumas already had more luggage space than the older trains they are replacing, but they have an extra 100 seats per train.
LNER said it was doubling capacity for large bags in four carriages in the Azumas by increasing the number of luggage stacks from two to four.
A third stack will be added in three other carriages on the nine-coach trains.
A total of 21 seats per train would be removed as result, but the modifications won’t be done until the New Year.
These will be among non-reservable seats at the ends of carriages which do not have windows - designed for passengers with light sensitivities or not wanting distractions.
By mid-December, around one in three of LNER’s cross-Border services to and from King’s Cross on the east coast main line will be operated by Azumas.
These include on its trains to Glasgow, Aberdeen from today, and Inverness from 9 December.
NER managing director David Horne said: “We’re pleased to be able to respond to customer feedback by increasing the space available for luggage on board our longer-distance services where customers typically travel with more luggage.”
Mr Horne said the move was the first major change to the fleet, which was launched as recently as May, and from Edinburgh in August.
LNER said: "The modification will install additional floor-mounted luggage racks for large luggage items where some non-reservable seats are currently located, offering extra on board luggage storage while not inconveniencing any customers already booked to travel.
"LNER’s nine-car Azuma trains will be fitted with the new luggage space in the new year."
The operator said the Azumas also had larger overhead storage racks than its older trains along with space under seats.
The planned upgrade comes as the train operator is looking to improve bike storage on the trains following criticism from campaigners Cycling UK, who describe the facilities as “downright dangerous”.