The choice of Rail Neutral Temperature depends on the region. In the US Midwest and plains it's typically around 95 to 100 degrees. In the UK it's stressed to 27°C (81°F), which is the mean summer rail temperature. In neither case is it the maximum possible temperature.
You don't want it to be the maximum possible temperature for the area, because a well-laid track can be in temperatures 40 to 50 degrees above the RNT before it starts to buckle, and a higher RNT is harder to achieve.
Nor do they lay the track or heat it at the hottest possible temperature. They achieve the RNT by stressing the rail. The RNT can also change over time, for example, if winter causes a curved track to become a bit more shallow.
I agree with you and with the linked-to author: I don't see how the proposed hyperloop system can ever economically handle temperature expansion, and the lack of a good explanation or discussion makes it easy to dismiss the seriousness of the proposal.
Then when it cools it pulls very hard, but because it has a tons of anchors on the ground it is able to handle the stress.
But it requires a very strong railway track. With the hyperloop the tube is not made that strong, and would tear.