Tram drivers didn't need (car) driver's licences until about, say, 15 years ago, when the media twigged and made a big deal about it, and the government decided that it would be a good idea politically (that's the way I recall it anyway) for tram drivers (who are driving on the roads, after all) to have driver's licences.

Train drivers, on the other hand, don't drive on the roads and don't need to know the road rules, so there is no reason for them to have a driver's licence.

Road rules on rails are simple: Road doesn't rule.