That's an over-simplification.
Getting a driver's job means passing a battery of psychometric tests and structured interviews. While the glib answer you may hear is that the railway is looking for "Steady Eddies" that does not mean that being introverted gives you an advantage as you may still flunk the tests.
What the railway is looking for in drivers are people who are confident, self-reliant and can work alone with a minimum of supervision while maintaining high standards. Driving can be a lonely job so it helps that you are comfortable with your own company, but that doesn't mean that you can't be an extrovert.
O L Leigh