To kind of repeat what others are saying, I've wondered if doctors saw soldiers with certain symptoms, and they were trying to figure out how to diagnose them, just assuming they must be seeing symptoms caused by physical trauma. So what could cause such symptoms? Well, perhaps the brain was damaged as in a concussion. So they reached for that explanatino.
Meanwhile, doctors were seeing other symptoms expressed by passengers in major railroad accidents, and they needed to find an explanation for those. Along came
railroad spine. Check out the link behind my words "railroad spine" back there--very interesting connection to shell shock. Again, there was the problem that not everyone was actually injured in exactly the same way--though a WWI soldier surely felt a shell blast at some point in the trenches. But it was easy to assume that the railway injured people in strange new ways unlike what had happened before, because it was strange and new form of transportation.
But until reading that wikipedia article, I didn't realize there was a connection between whiplash in cars and railroad spine. I just thought that railroad spine (unless there was an actual spinal injury) was PTSD, and whiplash (unless there was an actual neck injury) was somebody trying to get money from an insurance company, but now I don't know what to think. Are people with PTSD from car injuries (and wartime shells and peacetime explosions and railroad crashes) properly diagnosed now, and others with real injuries properly diagnosed also? Are there still people with PTSD/whiplash still not properly diagnosed. Ouch.
But anyway, my point is, I think like some others here that doctors were trying to find physical injuries from shell explosions first, and sometimes seeing physical injuries, but in the days before Xrays, CT scans and MRIs, nobody could be sure. Then later there was an understanding that sometimes it was psychological, sometimes physical, and of course sometimes (probably most of the time) there was both a physical and psychological component. I was just reading some old family stories about a Civil War vet and apparently he couldn't stand the sound of popcorn popping. Sent him into a cold sweat, the first few pops, the pauses, then more and more pops as the battle started in earnest. I'd read it briefly years ago, and every time I heard a skirmish at a reenactment, all I could think of was popcorn, and vice versa, hearing popcorn start to pop.