MARC train 538 from Washington shut down Monday evening, leaving passengers stranded in the train for about two hours.
MTA spokesman Terry Owens said the train, which leaves Washington at 6:13 p.m., "basically shut down."
"We don't know why it shut down," Owens said. "We assume it's weather related, but we don't know."
The train stopped just shy of New Carrolton on its way to Perryville.
He said that train officials gave out water on some cars. "We carry water on trains in the summer and if there is an issue we had water out."
In a statement, MTA Administrator Ralign Wells promised to investigate.
"I can guarantee you that a full review of all operations, communications and technical issues will begin immediately to identify the cause of this evening's problem and improve the response of both Amtrak and MARC in the future," Wells said.
Passenger Cheryl Harris, a government worker who lives in Baltimore but works in Washington, said the incident occurred about 10 minutes after the train left Union Station in the capital. She said the train shut down and the air conditioning system went off, prompting law enforcement officials to enter the train and open some windows.
At 8:13 p.m., about two hours after the train left the station, she said "They just turned on the air conditioner and people who were outside are now getting back on, but we're not moving."