The TSB says the investigators are there to collect new data as they try to figure out what caused the collision earlier this month.
It says three double-decker OC Transpo buses similar to the one involved in the crash have been brought to the railway crossing at the same time of day as the collision and under similar conditions.
Investigators are examining potential factors that could have affected the bus driver's perception and decision-making in the moments before the crash.
The TSB says those factors include the visibility of the train, the sound and visual cues of the crossing, alignments and sightlines from the road and potential environmental impacts on visibility.
Six people died and more than 30 were injured when the bus slammed into the train after going through a safety barricade on Sept. 18.
(CFRA, The Canadian Press)