What the Overground disruption is for
As I've posted about before, most of the London Overground network is going to be disrupted this autumn for engineering work. If you haven't seen it, the official leaflet about the replacement buses and changes to service is online.
But I wanted to go over what work they're doing and why, and how it will benefit passenger services. Basically, the problem is freight traffic, particularly container traffic from the ports around the coast of Essex and Suffolk. A lot of this traffic ends up going north on West Coast Main Line, and the only real way to get there is along the most congested part of the North London Line. Here's a diagram:
The current main freight route is in dark blue. Freight from the LT&S Line has to run across both pairs of tracks of the Great Eastern Main Line. Freight from both directions has to pass along the two track section of the North London Line from Stratford to Dalston Kingsland, which is also one of the busiest passenger sections.
It should be obvious from the diagram that it'd be much better to send freight from the LT&S Line along the Gospel Oak to Barking Line (shown in light blue), and that's exactly what they're trying to do. The problem that needs fixing first is that bridges and tunnels along the route aren't tall enough to allow modern containers through. During the closure three bridges over the Gospel Oak to Barking Line will be demolished, and the track in Hampstead Tunnel will be lowered.
Similar upgrades to other rail routes are planned to reduce the importance of the London Overground network to freight. Work to allow larger containers on the Felixstowe-Ely-Peterbrough-Leicester-Nuneaton will provide an alternative to the North London Line, while Southampton-Basingstoke-Reading-Oxford will provide an alternative to the West London Line.
The frequency increases on the London Overground network can only happen if freight traffic is properly managed, and these various upgrades are an important part of that.



