From - USER GUIDE for the operation of CENTRAL DOOR LOCKS on InterCity Slam Door Coaching Stock; BR33070/50 Issue 2, British Railways Board/Intercity, April 1996.
It may come as a surprise to you to learn that the external doors on passenger trains were not routinely locked - even while travelling at speeds of up to 125mph - until as late as 1994. This change in practise followed a series of unfortunate accidents involving passengers falling from moving trains in the late 80s and early 90s, though the problem had been ever present on the railway from the very beginning. On slam door trains the central door locking (CDL) consists of a pneumatic bolt which drops from above into a unit on the door to hold it securely closed, even though the external handle may be operated. As its name suggests, the equipment allows all the doors on a particular side of the train to be locked or unlocked by the guard from a single location on that side of the train. If you've ever travelled on an InterCity 125 type train (HST in the railway vernacular) then I'm sure you'll be familiar with the the reassuring 'clunk!' noise which comes from the CDL units just after the train stops at a station and again just before it starts moving. It is interesting to note that on conventional slam door trains there is no interlock between the door locking and the brakes or traction equipment, so the train can still move with the doors unlocked or, indeed, with the locking bolts correctly dropped but the doors still wide open. However, the train brakes will apply if someone uses the emergency egress handle while the train is moving.
This is a fairly brief publication at a mere 27 pages including appendices. It comes in a smart blue A5 sized folder which features both the famous Newell and Sorrell designed InterCity logotype and a rather oblique stylised keyhole motif as well as the rather long-winded title. The document features a number of full colour diagrams and photos and is printed on heavy satin paper. Evidently InterCity had money to burn!