FTA safety chief Joe DeLorenzo said safety briefings involving hundreds of employees must take place immediately because the federal agency has “determined that there is a combination of hazardous conditions and practices such that there is a substantial risk of death or injury.” , July 28 letter addressed to MBTA general manager Steve Poftak obtained by the Globe. Starting at 12:01 am on Saturday, all employees who operate or protect out-of-service train cars must review and discuss the events surrounding three recent train accidents, including the one at Braintree Station on the Red Line on Monday, according to the letter, and retrain on MBTA procedures. “FTA requires a safety prohibition to prohibit the MBTA from allowing any such employee who has not attended a safety briefing to move rail vehicles in yards or shops,” the letter said. The T must update the FTA every 24 hours from Monday on its progress to keep all affected workers informed. Within five days, the T must submit to the FTA its procedures for disconnecting and assembling trains, and within 10 days, the T must develop and implement a form to document the results of vehicle inspections. MBTA spokesman Joe Pesaturo said it will take hundreds of employees to receive the update, and it takes about 15 minutes for each employee. The T will make the safety updates on a rolling basis and expects the service “can maintain service at existing levels with little disruption,” he said via e-mail. Pesaturo said the MBTA shares the FTA’s concerns about “uncontrolled movements involving out-of-service rail cars.” “By fully supporting the FTA’s ongoing review of safety-related processes and practices, the MBTA is committed to providing the training and tools necessary for employees to create and maintain a safety-first culture,” he said via e-mail. The MBTA was already ordered by the FTA on June 15 to immediately address the runaway train problem by developing written procedures and training for moving trains out of service and a compliance program to ensure rules are followed. The directive was one of four issued by the FTA as it concludes a months-long safety inspection at the MBTA that began in mid-April. The federal agency has intervened locally in this way only once before and began monitoring the T after a series of safety incidents, including the April 10 death of a Red Line passenger at the Broadway station in Boston. Pesaturo said that following the FTA directive, the T has issued new “safety directives, training and [policies] for operators in relation to train movements in railway yards and rolling stock facilities.’ However, early Monday morning, a two-car Red Line train with “reduced braking capacity” rolled out of an MBTA train station and onto the tracks at the Braintree station when an operator lost control, disrupting the morning commute. The train eventually stopped about 800 feet north of the station, Pesaturo said at the time. No one was injured. The T has reported five train derailments to FTA in the past year, including Monday, according to the federal agency. One of those involving Red Line cars in December resulted in three injuries. The final report from the FTA is expected in August. Taylor Dolven can be reached at [email protected] Follow her on Twitter @taydolven.