View attachment 67296 Adam, thanks for the compliments on the video.
To be honest I don't know how they are going to modify the bridges. The ground work at the A602 bridge stop just short of the bridge supports (red line on the attached image). From my conversations with the supervisor this new section of track is going to be connected via point off the existing Hertford loop line whilst still retaining connectivity of the Hertford loop to the ECML as it serves a s a relief line and most of the freight services are routed this way so as not to bottleneck trains between Digswell viaduct and Woolmergreen. Looking at the support pillars and existing equipment as indicated by the arrows in the attached image, a lot of work is needed if a track running parallel with the ECML is to merge with the Hertford line.
The six-hill bridge should be easy, basically just mirror the existing lines and drop the OHL to get under it. Looking at the bridge itself, it looks as if they planned for an additional track when they built it given the span. When they built the new town 70 years back it seems they expected the additional platform and track to be needed at some point in the future and left enough ground to do so..

(well that's my thoughts).