I have been in the process of researching capsule pipelines, which include pneumatic mail and despatch systems, for a couple of years (on and off). Initially this was an academic exercise resulting in a paper on a potential modern application of the concept, 'The use of pipelines for the urban distribution of goods', (1998), Transport Policy no.5 pp61-72, with Michael Roe. I was rather shocked by the apparent lack of any 'standard text' on the history of the subject, and vowed to return to it at a later date.
My research (now as a private individual) is still ongoing, although quite a bit of material has already been collated, and put up on a website, vis:
The history pages should provide your project with some additional material you don't already have. I've had access to some of the texts held by the British Library - both the technology and the first applications came from Britain and there is a fair bit covered. This includes George Medhurst's original pamphlets and manuscripts (totally illegible) - Medhurst is reputed to have invented pneumatic capsule pipelines, although there is some evidence to suggest otherwise which I am still attempting to confirm. I still have difficulty believing that something so important to the communications network in most major European and Eastern US cities can have so little written about it.
I have found a number of items on your website of use, and so I feel I owe you whatever I've come up with. Please feel free to either copy text off the web site or maybe just register the web address.