Since no-one else has made any suggestions I'll offer a couple of ideas. The sign behind the party says Transport rather than Tramways which ties in with my thought that this is the late 40's or 50's from the clothes. There are only men (& boys) and the man on the right appears to be holding a length of rope or cable. Since this is also at the playground where the workshops were (are?), and the sign appears to have been chosen as a background rather than being accidental, could this have been a group photo of the staff? Perhaps marking the end of a piece of work such as re-cabling the power supply? Just a thought.