Nice shot's by the way, just to let you know, if you switch off the shadows, that grey box around the plasser will dissapear, also they very rarely use any plassers or tampers on over bridges, as the bridges cannot take the strains of the machinery shuddering over the track, it will be the equivalent of a minor earthquake to the bridge.
Most rail bridges can take the weight of the machinery going over a bridge, but they cannot take the shuddering the machine would give off, if it was working whilst crossing the bridge.
Hope this helps
Joe Airtime