Thanks to everyone who replied to my questions about keeping tunnel and bridge companies in the railroad database. I conducted a similar poll ten or twelve years ago and the answer was exactly the same. Most collectors want me to keep those kinds of companies in the catalog, so I will.
Collectors constantly send inquiries asking me to add all sorts of near-topic companies to the project. I completely understand their rationale, because that is why I started this project in the first place. Simply put, they have certificates that are not covered by any other price guide.
The problem is that adding new companies is a monstrous task and greatly dilutes my efforts at cataloging railroad certificates. Not to mention, that off-topic companies do not help the vast majority of collector who specialize strictly in railroad certificates. As a matter of policy, the only companies I now add to the project are railroad companies and manufacturers which were extremely close to railroading.
For those of you interested in my selection criteria, I ask you to visit my completely re-written page concerning Types of companies included in this guide. I have been fighting an exhausting battle against project expansion for fifteen years and this is the latest attempt at strengthening the lines.
(The picture at the top is from a stereo view card from around 1900. It shows a Pennsylvania Railroad bridge across the Potomac River at Washington DC.)