Unfortunately, trains are a different scenario than cars, mainly due to the fact that they are on rails and are generally kept well apart through rail signals.

A couple seconds where a train driver is distracted != a couple of seconds where a car driver is distracted. A button is not fast enough (you would need to press it >once per second, which is not particularly comfortable)

This is why it is "necessary" to have more "advanced" ways of detecting driver inattentiveness.