The LNER built these ten corridor tenders, numbered 5323 to 5332, in 1928 against tender order N°. 50. The purpose of this type of tender was to allow the Flying Scotsman train to run, without stopping, between Kings Cross and Edinburgh, and yet allow the relief crew to travel off the engine footplate. 

When new, these tenders were coupled to Class A1s with long travel valve gear and to Class A3. From October 1936 these tenders were modified and then coupled to A4s No's. 4482-90/98. The last corridor tender was removed from an A3 in September 1937. 

 The modifications for the A4 class consisted of: 
(1) Replacing the original spoked wheels with disc wheels. 
(2) Increasing the width between the side sheets at the front end. This necessitated moving the front handrails towards the rear. At the same time, on the tenders behind 4488 to 4490, these handrails were increased in length by 3" to 4' 6". 
(3) Raising the height of the front and division plates to cab height and providing a fairing. 
(4) Removing the second lamp bracket on the left side of the back. 

In preservation, the modified A4 type tender is coupled to 4472 Flying Scotsman and to the preserved A4s, 4488 Union of South Africa & 4498 Sir Nigel Gresley. 

For a more detailed history of these tenders Part 2A of Locomotives of the LNER published by the RCTS is essential reading. Other valuable sources of information and photographs are: 
For original condition (A1/A3) 
Isinglass Drawings Drg N° 33 
Locomotives Illustrated 25 - Ian Allan 
The Power of the A1s, A2s and A3s - J.S.Whiteley & G.W.Morrison - Oxford Publishing Co. 
The Gresley Pacifics - O.S.Nock - David & Charles 
Yeadon's Register of LNER Locomotives - Volume One - Irwell Press 
LNER Reflections - BBC Hulton Picture Library - SLP 
For modified condition (A4) 
Isinglass Drawings Drg N° 317 
Yeadon's Register of LNER Locomotives - Volume Two - Irwell Press - Ian Allan 
East Coast Pacifics at work - P.N.Townend - Ian Allan 
The A4 Pacifics - P.N.Townend - Ian Allan 
Locomotives Illustrated 38 - Ian Allan 
The Power of the A4s - Brian Morrison - Oxford Publishing Co. 

Original condition A1/A3 
Wheels 4’2” diameter, 12 spoke. The closest available is 4’3” 12 spoke with 5/32” plain axle, Slater’s Ref 7851MF 
Modified condition A4 
Wheels 4’2” diameter, disc. The closest available is 4’3” disc with 5/32” plain axle, Slater’s Ref 7851GMF