It's a little more nuanced than that. Old cars are essentially a flat base with a structure stuck on top. This forgoes crumple zones, yes, but it also results i

It's a little more nuanced than that. Old cars are essentially a flat base with a structure stuck on top. This forgoes crumple zones, yes, but it also results in a situation where one car body climbs over the other, as observed, causing the structure to shear in two. Anti-climb bars are a key technology in stopping this and were introduced after a number of large accidents where two passenger cars essentially became a pair of scissors and slid into one another. It also means that the cars stay roughly in line, ensuring that the reinfroced parts of the structure stay bearing on one another and transfer the load into the body rather than into the weak cabin area. If you think about it, the structure is very strong when pressed end-on, but as soon as one climbs it's essentially crushing from above/below and the structure is far weaker that way. Outdated regulation plays a part, particularly around the assumptions of where loads come from. As you mentioned, the buff crushing load assumes a longitudinal action only. Moreover, there has historically been minimal roll-over strength in cabins to protect pasengers. Change is disappointingly slow, however, and with asset lives of 50+ years due to the tough financial challenges facing particularly passenger rail, it takes a long time for new regulation to come into practice.