I'm sure you've all heard of the anecdote with these machines (can't remember if it was 21 or 29, mind). They were regularly used on banking duties out of a Glasgow station (forgot which one), and due to their unreliability, they had to be coupled to the train they were pushing in case they went kaput. If they got to the top of the bank without dying, the driver pulled a length of wire in the cab that was attached to the coupling in order to uncouple. Now, I can't remember where I read it.
I concur with Daddyman on the subject of archives. My experience of using them over the years is that items are either not catalogued or wrongly catalogued. Sometimes just asking for XYZ will produce a negative reply, when it actually exists - filed with something else. You might need to ask for a broader search term and hope the plans you're after are in it. Or you might have to request to trawl through the whole f**king lot. A good archivist knows *everything* about *everything*, a bit like editors at publishing houses. This hefted knowledge has largely been lost over the years due to staff cutbacks.
The chances are that the NBL loco drawings are in Glasgow - you just need to strike lucky in speaking to the right person. Or make a high-level request for someone from your firm to spend time in the vaults and thus bypass the catalogue.
RWJ