Finally my first post in this thread! I have recently survived bootcamp, and while that is a breakthrough in its own way, I have another story to share.
The setting: train from Aberystwyth to Birmingham, I have settled down in one of those seats at the end of one coach, those with some extra fold-away seats running rectangular to the one that I was occupying, and there I placed my luggage.
As the train comes to a stop in Machynlleth, a young man boards the train, with some equally unwieldy piece of luggage, accompanied by a woman who I (correctly) assumed was his mam.
I rearranged my luggage so that there is enough room for the young man and his luggage, and while doing that, I clearly heard the two saying their goodbyes yn y gymraeg.
So, after the mam left, I asked him "Ti'n iawn fan'na?" (You okay over there?) -- which sparked a mostly-welsh conversation (about 90%) that lasted pretty much the whole way to Birmingham, where we both got off the train.
Although he will probably never see this: Thank you, Matthew, where ever you might be now, for giving me this unexpected extra chance to use cymraeg "in the wild"!