Charles Ellson wrote:
On Mon, 16 Jan 2017 21:57:06 +0000, Martin Coffee
wrote:

On 16/01/17 19:49, Basil Jet wrote:

The new bridge allowing room for OLE on the Goblin opened a while back.

Given that cost-cutting measures on the Borders Railway in Scotland led
to it having several bridges that only allow a single track beneath
them, I was astonished to see that the Wightman Road bridge looks to be
long enough for three tracks.

Watching videos in Youtube, the adjacent ECML bridge has a span over
both Goblin tracks and an extra unused span on the north side seemingly
wide enough for an extra track, and the old Wightman Road bridge had a
single three-track span aligned with the two spans of the ECML bridge.
So the new bridge is as long as the bridge it replaced.

It's hard to believe that a bridge 50% longer than it needs to be would
have been cheaper than just filling the unused width with soil or even
concrete. So I'm wondering why did they build the long bridge? I'm also
wondering why the unused span was ever there.

It would be great if the answer was that they are planning a Goblin
station immediately west of the ECML and the span was left to allow
pedestrian access from Wightman Road, but I have heard nothing along
those lines. The current walk from Harringay to HGL is far enough and
lonely enough to deter anyone from using it as an interchange.
The answer is simple.

If the trackbed was blocked by the bridge and Network Rail subsequently
decided to add the third track then the highway authority would be
obliged to modify or rebuild the bridge to allow the additional track.

That presumes it is the HA's bridge not NR's.
The more simple answer is possibly that infilling the unused width is
not the most desirable option as it introduces a physical feature that
didn't exist before and which could invite future trouble. If the new
span is more or less sitting on the same foundations as the old one
then the structural changes will be less than if the bridge had been
shortened and the spare bit replaced by a different structure.

Yes, that makes sense. Not having to build a new abutment probably saves
more than having a shorter span.

Plus, if both freight and passenger services on the newly electrified line
grow strongly, having room for third track sections will be useful
insurance.