Never expire vehicles
This setting will stop vehicles from going "out of date" and so they will remain available until the very end of the game.
Normally, vehicles have a limited shelf life as well as a limited in-use life. When their shelf life expires, they are no longer available to purchase or replace and any vehicles of that type in play will slowly deteriorate.
This setting makes sure that vehicles are always available. Thus, you can keep a Kirby steam engine and run it next to a shiny new monorail.
If you want to get the already expired vehicles back then make sure that this setting is enabled, open the console and type
[patches] never_expire_vehicles = true
Note that the reliabilty of the vehicles is not affected by this setting, so the older vehicles may be quite unreliable.