When temperatures top 90 degrees or heat up dramatically in a short time frame, train tracks can expand. That means VRE trains run about 10 mph slower.

The VRE on Wednesday announced one of its first heat slowdowns of the year. From 1 to 7 p.m., passengers can expect trains to arrive progressively later as they travel down the Fredericksburg line, starting with a back-up of just two minutes at Franconia-Springfield and expanding to an 11-minute lag by the end of the line.

For a full explanation of the science behind heat delays, read the VRE’s post here.