Some companies follow a Welsh Language Policy in Wales. As I understand it, these are a set of guidelines that a company agree to provision, but not all the guidelines are required to be followed, and it appears that a company can also determine for themselves how much of certain guidelines are followed. It's apparent to me that these Policies are more like an agreement than a law-binding document, but I'm not entirely sure about the legal standing of these policies.
The bus company I work for has such a policy, which they generally follow (correspondence when requested, social media posts are bilingual, notices are bilingual etc.). But, we are not required to be Welsh speakers ourselves, although the use of Welsh where we can speak it is certainly encouraged. Trenau Arriva Cymru do offer Welsh information and signage on their trains and at their stations, as they do also on their buses in Wales - something that some bus companies in Wales, big or small, still don't do. I cannot speak for all of their staff though in terms of them being speakers.
With all these new schemes where Welsh is being encouraged (Million speakers by 2050, for example), and the rail franchise in Wales up for renewal next year, it would be an idea if one of the prerequisites of the new franchise will be the inclusion of Welsh information given on public announcers as a compulsory measure, and a mix of staff who are able to speak Welsh. Of course, care needs to be taken here, since making the ability to speak Welsh compulsory in offers of employment could be seen as discriminatory towards non-Welsh speakers.
So no, it's not a requirement anywhere on public transport systems in Wales for its staff to speak Welsh. But it is very much encouraged by many companies here.