I have thought about this as well. There are at least two issues:
1) The number of trains on the track would increase significantly making section control even harder. (In Germany this is one of the limiting factors for the overall capacity)
2) In order to attach/detach the front and back of the trains would have to be straight and vertical. At higher speeds this would lead to aerodynamic problems. You can see this issue when comparing local commuter trains which are vertical and often can be linked this way and high-speed trains which tend to have special aerodynamically shaped noses. The higher the speed the more elongated the nose.
This change can be seen in the famous mosaic showing Alexander the great on horseback and the Persian king in a chariot. The added flexibility and mobility gave Alexander's army a significant advantage.
In one case I came across a premature engineering sign-off to confirm production readiness could be averted simply by asking the manager (who himself had the required credentials) to sign off on the change himself.
1) The number of trains on the track would increase significantly making section control even harder. (In Germany this is one of the limiting factors for the overall capacity)
2) In order to attach/detach the front and back of the trains would have to be straight and vertical. At higher speeds this would lead to aerodynamic problems. You can see this issue when comparing local commuter trains which are vertical and often can be linked this way and high-speed trains which tend to have special aerodynamically shaped noses. The higher the speed the more elongated the nose.