There are many theories that try to explain
how this weakness caused the bridge to collapse.
Bill Dow, who still lives in the area,
has made it his lifelong study.
He's spent decades analysing hundreds of pages of reports
and correspondence from the time of the disaster.
I suppose nowadays we tend to think of the Lockerbie disaster
where in fact every single bit of that aircraft was recovered
and then was reassembled on a framework back in Farnborough.
Nothing like that happened in the Tay Bridge disaster.
When you do not gather the detail,
you cannot explain all of the things which actually happened.
Bill's theory revolves around the train derailing.
The dimensions of both the carriages and the bridge
were such that it would be very easy to get a very high speed wind
going over the tops of the carriages.
Whereas the same wind, as it entered the bottom of the carriages,
it would be impeded by the wheels,
the brake gear, the steps and all that stuff.
This is exactly what you get in an aircraft wing
and the result is that you get a lift from this.
It was a strong wind like this
that caused two girders to fall into the river during construction.
Bill Dow has discovered that one of these toppled girders
was nevertheless reused in the finished bridge.
Any girder which has fallen and which has been straightened out
is quite naturally weaker than any other.
Also there's the tendency for it to go back to the shape which it had
when it was bent.
Something must have happened within that girder
because the various plate layers and surface men describe the engines
as nodding into the girder.
The descriptions suggest that there was a slight change in direction
in the rails as you passed from the low girders into this high girder.
And that change was reasonably sudden
just at the point where the two girders met.
And with that wind blowing,
and the combination of the kink in the rails,
was such that the second-class vehicle came off the line.
Once the second-class vehicle was off the line
it would roll along the floor of the girder quite well
at the same speed as the train.
But it couldn't pass between the fourth and fifth girders
because there was a triangular plate there.
It would have hit this plate with a severe jolt.
This jolt would have cracked the cast iron lugs
but it would have had very little effect on the wrought iron.
A sudden impact is the key to his explanation.
It was accepted at the time that the cast iron lugs were a weakness
but Bill thinks they broke as a result of a shock wave
down through the cast iron.
This could be true
but modern engineering requires explanations
with the highest possible levels of evidence and analysis.
