How can I retire multiple vehicles?

marking multiple vehicles as Out of Service to retire them without deleting

If you need to retire several vehicles but do not want to delete them, there's a utility for that.

Go to Utilities, Vehicle Utilities, Vehicle Options Utility

Choose the vehicles you wish to retire on the grid, then click the Edit Selected (pen and paper icon), choose Out Of Service and set it to Yes, the click the Save icon to retire the selected vehicles.