I am not knowledgeable about this, but if there is a signalman in the picture I would think he would be the third from the left. Three appear to be painters or carpenters, the man on the right is probably a ganger (or someone who works in the yard) and the man on the left looks as though he only has a simple uniform, as would befit a junior porter. Of the other two, the one to the right of centre has a watch chain, which a signalman wouldn’t need as signal boxes have clocks. Which leaves the man left of centre as the most likely to be a signalman, although the man with the watch chain is still a possibility.
Someone in the Midland Railway Society would probably know more about uniforms.