Short, but dramatic.

It gets hot, the rails want to expand but since the rails are clamped down to the ties, nothing can happen…until the ties start sliding. The rails are apparently welded together, and there must be a good reason for doing this and not leaving an expansion gap every so often — rails weren’t always welded. It may be that a gap causes problems for higher speeds. Or maybe it’s as simple as shorter lengths of rail being too easy to steal, and the disaster that ensues when a train encounters a missing rail.

OK, wikipedia tells me maintenance is cheaper and it allows for higher speeds and a smoother ride. Plus they mention flash butt welding. Tee hee.