Prior to the use of continuous welded rail there were gaps between each 39 foot section of rail.

Nowadays with welded rail, careful compliance with construction standards is required. Slow orders are issued during heat waves and frequent track inspections are conducted to look for potential or actual sun kinks.

A good summary of the issue can be found in the Heat Orders FAQ from Virginia Railway Express. [1]

[1] http://www.vre.org/feedback/frequently_asked_questions/faq_h...

The gaps between each section of rail can be quite large.