Since the Cardiff Area Signalling Renewal (CASR) project that completed in 2017, Queen St to Central has been bi-directional and Valley lines trains can depart or arrive Central to Queen St from platforms 4, 6, 7 or 8 at Central, although P4 at Central is rarely used for movements to/from Queen St.
Given the stone viaduct extends under the o/b quite a few metres more the majority of the o/b is sitting on top of the viaduct at that point although in decreasing proportion as the o/b traverses over the main lines. It would be deriving its integrity from transverse members so potentially a failure there in a hidden part of the structure. It strikes me also they've loaded that edge beam with additional metalwork im surmising to provide protection against the new electrification wiring below as well which maybe a contributory factor.
Who responsible of the structure there .now transport for wales or network rail
The affected bridge is right on the boundary - in the Network Statement it says that the boundary is the "intersecting bridge", which is the one in question. Openrailwaymap shows the boundary as being in mid-span of the bridge, and the approach ramp from Cardiff Central belongs to Network Rail.
The western online sectional appendix shows the route boundary north of the main lines at 0m 13ch, ie the entire bridge belongs to NR. As you suggest, having a mid-structure interface makes no sense.
The 5 miles strongly suggest the bridge is from 0m 10ch to about 0m 18ch. The quail gives 10ch and 19ch as the bridge limits. It's not clear though exactly where the route boundary is in relation to the boundary of the metal bridge vs the brick approach ramp which is likely included in the bridge limits shown on the plans I can see.
An unfortunate use f the word "fell" !.
The conspiracy theorist in my feels it is likely that TfW and Network Rail have been assuming the other party is responsible !.
Looking at the satellite view, the lines leading to Central and Bay seem to be on the same structure as they approach from Queen Street and split onto separate structures right above the main lines. If TfW own the Bay branch and Network Rail own the rest I can see the situation is complicated, to say the least.
Indeed, it is the now well known (within railway circles) Intersection Bridge, which was the pilot for the engineering solution to reduce the cost of electrification by not having to demolish overbridges. A pilot which was almost forced by necessity, as the particular constraints of the location (I believe a canal in the vicinity doesn't help) meant either raising the bridge or lowering the main line tracks would have been (if possible) very expensive and hugely disruptive.
Bear in mind that was the first time it was used, and from all the articles I read, it was very much regarded as a prototype. It also came at the tail end of the electrification project, being at the end of the line. So I'm not surprised that it was the only time it was used on the GWML project. Let's just say it's good news that it is now being used more extensively on the MML, and helping to keep the costs of that project down.
Maybe experience on the TAM lines has led the engineers to conclude that the latter is the better approach, either in terms of cost of installation, ongoing maintenance, or there is some other benefit of minimising PE sections if at all possible.
Not saying that's the case here, but certainly something that needs to be considered.
Many highway bridges crossing rivers are in that situation as the middle of the river is often the county boundary. Each county is the Highway Authority each side and therefore share responsibility for the structure.
That was the route of the Newton le Willows to Newton Abbott Motorail (not reversing at Newport) was it not?
looking at Open Train Times, the only reversal available at Queen Street is for trains from Cardiff Central or Cardiff bay in the Southbound loop platform.
