I think you are right and the expansion joints at each pylon would not be too much of a problem but getting hundreds of miles of tubing to smoothly expand and contract each day with out the slightest bit of buckling is not something I can imagine working in real life.
The expansion/contraction would have to be actively assisted by small motors on some pylons, particularly when you take into account the added tension/compression due to elevation changes. Friction over 300 miles of pylons is just too severe for the expansion to happen completely passively. You also don't want the tube randomly "caterpillaring" in one direction (northward or southward) over time, and if the tube were somehow severed at the top of the Grapevine, you can't have it sliding down unchecked in both directions! But a distributed solar-powered motor system for this is straightforward enough, and it probably could be set up to act "locally" and autonomously, with occasional recalibration, so no central system would be required to synchronize the action. (and thus there would be no single point of failure.) Such motors would also be required if local sections of the tube need to be disconnected for repair, maintenance, etc.; passive forces would otherwise pull the ends apart rapidly! (and/or slam them back together again!)
