Questions around responsibility spilled into another day as opposition politicians, who have called for Mr. Vaishnaw’s resignation, accused Prime Minister Narendra Modi’s government of not doing enough to ensure rail safety.

“Consistently flawed decision-making has made traveling by rail unsafe and has in turn compounded the problems of our people,” said Mallikarjun Kharge, an opposition leader in Parliament, in an open letter to Mr. Modi, adding it was “incumbent upon the government” to determine the reasons behind the incident.

Railway authorities have asked for the case to be handled by India’s top investigation agency, the Central Bureau of Investigation, Mr. Vaishnaw told reporters on Sunday. The agency typically handles high-profile criminal cases in India, including fraud and corruption, and details were not offered on why it had taken on the case.

Officials have focused on the malfunctioning of an electronic signal system designed to stop accidents as a cause for the crash, but have not ruled out further sabotage. Mr. Vaishnaw told reporters on Sunday that he would leave it to investigators to share further details.

More than 20 million passengers take India’s trains every day, and the rail network is one of the largest in the world. Mr. Modi’s government has in recent years presided over an overhaul of the network, dedicating an increase in spending in the last fiscal year and unveiling a fleet of new electric trains. But a recent audit found that spending is falling for safety measures, including improvements for more than 13,000 older trains and track maintenance.

Improvements had in recent years been made for rail safety in India, with the number of serious train accidents dropping. Rail safety had appeared to improve in recent decades, with the number of derailments and serious train accidents falling.