The National Transportation Safety Board (NTSB) has started an investigation into the CTA Yellow Line train crash that occurred in Chicago on Thursday, injuring nearly 40 people. The crash happened when the train rear-ended snow-removal equipment on the same tracks. NTSB Chairperson Jennifer Homendy stated that a team of investigators has begun documenting the scene and expects to spend about five days there. While the investigators have not yet reviewed event recorders or video footage, they plan to issue a preliminary report in a matter of weeks. However, a final report with an analysis of what occurred and recommendations may take up to 18 months to be released. It remains uncertain whether the train operator saw the equipment and tried to slow down or stop before the collision, as well as whether they had been warned about the presence of the equipment on the tracks. The NTSB will also investigate the role of the automatic train control system in the crash. The CTA has not provided further updates on the incident.