Mexico City "Subway" Collapse
This seems to be the location of the collapse. The failure appears to have been at mid span. The columns seem to be intact. The failure location seems to be in the last span of 2 sets of rail. The very next span begins the transition to 3 set of rails.
Those beams look rather shallow to me. There are plenty of stories on line about how the whole line showed damage when inspected after a 2017 earthquake, but not much was done about it.
Let's see: The failure happened under load and mid-span. To me that might exclude failure to due shifting piers or span seat bearing points because of a seismic event. My bet is structural failure of a beam, especially since it occurred at mid-span.
I hope the investigators get to the bottom of it without undue pressure or influence from others.
When I lived in Mexico, I was studying Civil Engineering at ITESM in Monterrey- a very good university. I took concrete, reinforced concrete, and pre-stressed concrete, one each semester. They build a LOT of stuff out of concrete, and they of course have major seismic issues. So I think they know what they are doing with respect to design. However, I heard several people mention that CORRUPTION may be the problem. i.e. someone cut back to save money, or didn't build it according to the design, or used a weaker concrete mix, etc.
I go to Mexico a few times a year, was just there last week, and I can tell you that corruption is rampant in the country. Money talks, and bribes are common.
Last weekend when I was there I had lunch with an old friend who was dean of engineering at the university (now retired). He was my dad's Phd student (EE) here in the US back in the early 70's.
I asked him what an engineer makes, average is about $15K/year. Start under $10K. But I do believe they are well trained and knowledgeable.
I looked at a few apartments in high rises in Monterrey (my daughter has been there for 13 years and now has a son) to stay at when I go down, nice ones for about $1000 to $1250. My wife always spends a few months a year there, probably will more now that we have a grandson there. But, I'm not so sure I want to live in a high rise that could possibly be sub standard construction. But at least it is not seismically active in that area.
Monterrey has a fairly large population of upper middle and upper class people, but also a very large population of low income people as well. Huge wealth gap. So there is a lot of reasons for the corruption. I can't tell you how many times I had to give cops $1 or $5 while I was there in lieu of tickets. They would stop me for no reason since I had Pennsylvania plates. They had to pay daily "rent" for their jobs up the line. The police chief in Mexico City when I lived there was totally corrupt. From wikipedia...
Durazo converted the police into a racketeering empire Although he earned less than $1,000 a month, he acquired two palatial homes, a collection of vintage automobiles and properties in Canada and the U.S. Durazo amassed a fortune in illicit wealth from the bribes paid by every cop in the city, the cocaine trade and kickbacks on the purchase of police equipment.
Having had a "near miss" myself, it could be a case of steelfixers being used to a routine, followed by poor inspection. Most of the spans would have had a particular size of rebar along the length, so everybody was used to that. Where the third track starts a heavier bar would have been designed under that area, say 1.5" instead of 1.25", but it was overlooked (perhaps steel fixed at the end of the week and everybody more concerned with going home). Being the bottom layer of reinforcement, once the rest went in then it may have been overlooked by inspection (Yes, it shouldn't have, but it does happen, standing on top of the rebar cage the size difference in the bars wouldn't have been too noticeable). At the end of the job somebody should have registered that there were a couple of bundles of the larger bar lying around unused - that's what happened on my near miss.
Once concreted and in position the span would be overstressed from day one: each train causes flexure and fatigue would quite rapidly build up to the limit the smaller bars could take.