Northolt Tunnel - HS2

[Matt P]

With the benefit of hindsight , particularly the issues HS2 later encountered with Phase 2, or perhaps even should there have been better foresight, was the Northolt Tunnel necessary?

The tunnel replicates the route of the largely unused former GWR mail line to the Midlands. I have not researched this in too much detail, but I understand the reason for the tunnel is that it was considered less disruptive that reinstating the existing railway. How true is this?

The existing railway corridor would undoubtedly need to be extensively reconstructed, although that doesn't look like it would be prohibitively expensive in itself. Indeed less difficult than reopening entirely abandoned railways. Electrification would pose challenges, particularly with clearances for bridges over the railway. But (relying on Google Earth admittedly) there aren't that many. This has also been done when electrifying other routes.

Although the alignment looks relatively straight, I have no idea what a maximum speed would be or whether reopening as a true high speed line would be possible. However, it presumably could be used as a conventional route, engineered for 125mph.

I accept that as a percentage of the total cost, Northolt represents a relatively small part of the HS2 phase 1 total. However, does that make it the right solution to have gone with?

 

[HSTEd]

The Northolt Tunnel was required to avoid a ludicrously expensive and disruptive reconstruction of Hangar Lane Gyratory.

The original plan was eventually coated as being significantly more expensive than the tunnel.
The targeted GC loading gauge, with modern electrification clearances had no chance of fitting under the existing bridges.

 

[LNW-GW Joint]

There would also have been complications with the parallel LU DC line, and at Northolt Jn where the Marylebone line comes in, and at West Ruislip LU depot.
The residential areas along the route would have needed extensive visual and sound-proofing, and there may have had to be "green tunnels" along the route.
The tunnel is more than half built now, so it's too late for a "what if".
It was originally due for completion around now, so is substantially late in construction.
We will probably never know the actual cost of the Northolt tunnel in isolation, but I expect it will have exceeded the original budget, like most of the project.