This doesn't look all that exciting to me...

... but it's what came along as the 4.20 from Nant Gwernol to Tywyn Wharf. I presume it's one of their diesel engines that they don't normally use for passenger service.
It's my fault that I missed the earlier steam engine going the other way. I messed around taking pics of bits and pieces and by the time the train arrived my battery died. Lesson learnt maybe.