There needs to be a way to stop players from entering, either directly or from another car, selected pieces of rolling stock. This would be done with a keyring-type item or similar that allows a player to "claim" newly created locomotives or cars so that only one or more specific players can enter them.

Separate whitelists could be added, one for players that can alter the whitelist (in the case of buying/selling IR stock) and one for players that can enter the piece of rolling stock, but not interact with its controls or contents. The latter is especially useful for trolleys, railcars, and any rolling stock that has both a cab and passenger area.