I originally built my inventory as a spreadsheet in Excel, but eventually I exported it to Google Sheets so I could see it (and edit it) on my phone. That's very useful when I'm at the hobby shop or a train show and I want to know if I already have a certain car number when several might be available. I have also added a wish list tab to track what I'm looking for, so shopping is a little easier. The majority of the other tabs are used to sort or filter the data in the master sheet, like a tab listing only cabooses or all intermodal equipment. Another tab lists industries and which cars are suited for those industries. Similarly I have a tab listing different trains and the cars that are specific to those trains. It didn't take long to realize many of the columns that were useful to categorize freight cars didn't really apply to locomotives and vice-versa, so I created a new spreadsheet for my locomotive roster. The locomotive roster sheet uses conditional formatting, so certain criteria will cause the row to change color. This is useful to pick out at a glance the locomotives that need decoders, already have them or have sound decoders, for example. Again the best part is these files are easily accessible and portable. I can even share them with my friends.