Deep underground at Walthamstow fiery welders fill tunnels with smoke while the din of hammers deafen workers in a hot cramped working space. This is the Victoria line upgrade at work.

The Victoria line has been progressively upgraded over the past few years, to deliver an eventual 40 percent capacity increase on the line north of Seven Sisters, compared to its older self, and now one of the last major engineering tasks is underway.

An £11 million upgrade.

To explain for those not familiar with the Victoria line, trains run from Brixton up to Seven Sisters, and there just 2/3rds of trains run past to the end of the line at Walthamstow Central.

The reason why some trains stop at Seven Sisters is thanks to an old railway junction in the tunnels just outside the terminus station.

When built, it was suitable for needs, which was a Victoria line with far fewer trains that traveled somewhat slower than today. As a result, they can only run 24 trains per hour over that junction due to its speed limits. The rest of the line aims to reach 36 trains per hour — one of the most intense rail services in Europe — but can’t with that old railway junction snarling things up at the end of the line.

Unfortunately, replacing the junction wasn’t just a case of removing bits and replacing them, over weekend closures, but required a complete gutting of the railway track and the concrete bed it sat on.

This is because the new junction is a totally different design. It’s longer, by around 6 meters in length, which could only just about be squeezed into the tunnel. A longer crossover means that the curve from one side to the other is more gradual, and that means trains can pass over it at faster speeds.

The older junction was also made from wooden sleepers cast in concrete, which was a maintenance nightmare, while a small hump in the centre of the junction further slowed trains passing over it.

To rip up an entire section of track, and the concrete bed, pour new concrete and lay new tracks is not a weekend job, and that is why, as you are bound to have noticed, the Victoria line is closed beyond Seven Sisters at the moment.

Long before the works started, a new yellow gantry crane structure was installed into the crossover tunnel, to help with the removal works — and to assist with future works, it is remaining in place as a new permanent addition to the tunnel.

Since the closure of the line, it has taken a couple of weeks and around 100 staff working in the tunnels to remove the old tracks, and break out the old concrete bed, and over the past weekend, the new concrete bed was poured.

In itself, a significant effort took place, to coordinate the arrival of 35 delivery lorries in one day to permit a continuous pour lasting nearly 20 hours. Just getting the trucks in time required support from police at times to keep junctions clear and at Walthamstow Central, it was poured down the ventilation shaft and along a 500 metre pipe to the junction site.

Two days of waiting, and the concrete is strong enough to work on, and there has been a logistical challenge to then bring down replacement rail junctions and equipment to rebuild the junction again.

Some new innovations include the switching gear, which thanks to the longer junction needs two additional support arms to push the tracks into place, and three more sensors to check that such a shift has taken place correctly.

They are also using a hybrid aluminum/steel design for the power supply track — as it reduces electrical resistance, and the associated loss of electricity as heat. Also thus lower costs due to less electrical losses.

At the moment, the works are on schedule, and being controlled from a room next to the station platforms, where usefully they were able to plug into the station’s CCTV system to keep an eye on the site.

It’s anticipated that scissors track crossing junction will be completed on time for the line to reopen on Sunday August 30th.

Once that happens, the final works can go ahead to enable up to 36 trains per hour to run along the entire length of the line. Not only more trains, especially north of Seven Sisters, but also less overcrowding in Central London from people waiting on platforms for trains running past Seven Sisters, as almost all of them will now do so.

When all the works are completed, trains should be running along the line with gaps of around 100 seconds between them, thanks to a new crossover junction that hardly anyone will ever see.

Site access thanks to London Underground, and John Hardy, head of track programme for London Underground.

Some more photos: