Commuter rail trains on the Framingham-Worcester line have been ordered to slow down a bit because of the hot weather, the Massachusetts Bay Commuter Rail Company announced.
Heat-induced speed restrictions are expected to cause delays of 10 to 15 minutes between 1 and 7 p.m. on Thursday, MBCR officials said in an e-mail advisory to customers.
The restrictions limit trains from traveling faster than 40 miles per hour, the advisory said.
The restrictions are normally seen when temperatures top 90 degrees or when high temperatures are sustained over several days. Heat causes steel to swell, which can cause a railroad track to kink or bend, posing a derailment danger.
MBCR, the company that runs commuter rail trains on behalf of the MBTA, imposed the restrictions Thursday.
"Heat restrictions are necessitated based on the difference between the temperature at which rail is originally laid versus the current temperature," company spokeswoman Rhiannon D'Angelo said in an e-mail. "If rail is laid at cooler temperatures, it is more apt to expand when the outside temperature increases than it would if it were originally laid at warmer temperatures."
The state bought the Framingham-Worcester line tracks last fall from transportation company CSX. When CSX had owned the tracks, they would order heat restrictions based on forecasts, a practice that at times led to the implementation of heat restrictions that proved unnecessary.
MBCR is now in charge of ordering the restrictions and does so on a case-by-case basis depending on the actual temperatures instead of predicted temperatures, D'Angelo said.
Thus, officials have not announced whether similar restrictions might be imposed over the next several days. Temperatures are expected to hover near 90 degrees through Sunday.