Over the years your vehicles will get old and decrepit compared to new models that come out. You could try replacing all of them one-by-one manually, but there is a much easier way to do it. Go to one of your company's vehicle lists by clicking the train, truck, boat, or ship icons at the top. At the bottom of the new window you'll see a drop-down tab called "Manage list". Select the Replace vehicles options to bring up the vehicle replacement window. From here you can automate replacement of vehicles in a particular group or type. Once you select what to replace them with, click the "Start Replacing Vehicles" button. The way that this works is that whenever a vehicle of a certain type or grouping enters a depot (usually for servicing), it will automatically be replaced with whatever you chose, as long as you have the money required to do it.