Two people sadly died in the collision at the level crossing in Barns Green this morning (Saturday, February 17).
Officers are asking anyone with information to get in touch by calling 0800 40 50 40 quoting reference of 07/02/18.
Information can also be sent via text to British Transport Police on 61016.
The Rail Accident Investigation Branch have been notified of this incident, the spokesman confirmed.
Trains are unable to run between Horsham and Barnham due to the incident, a National Rail spokesman confirmed.
Disruption is likely to be in place for most of the day.