If workers in critical infrastructure would get so high wages through striking, why don’t the train drivers have that already?

Because they're currently not working in critical infrastructure. With our current car based system the trains failing isn't that much of a problem. If a company really needs something to be delivered, they'll never ever transport it via train. They'll pay extra for it to come via lorry, because that's more reliable (and often also cheaper). And of course most people still have cars, so private transportation is covered as well. But if we manage to get away from car dependency to avoid missing all carbon emission goals, trains will become critical.

Besides that, there's also their employer. If the police went on strike, they'd be up against an entity that has almost unlimited funds (in relation to their wages) and that is willing to pay almost anything to prevent chaos in the streets. The state tends to be willing to pay for things that benefit the entire population. But Deutsche Bahn is more or less run as a private company. Hence most of the losses in question aren't paid for by Deutsche Bahn but by the rest of the population. So from their perspective it doesn't make sense to pay up.