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.
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.