In addition to sibling: the bike is, for many people, already, 'too cheap to meter', so the tax advantage goes to the train subscription. And quite some employers either give you a train subscription, or let you deduct kilometers travelled at some rate. That rate depends on modality, but the subscription for free is usually (always) a much better deal, _if_ you really don't need that car.

Whoever is left driving cars (which is, also in the Netherlands, a large fraction), can't for instance drive up to some remote train station, and rail the rest. That's how you can get those who still drive out of their cars; those that already take trains are covered by the rules as they are.