SCRAP railway sleepers and timber were set on fire at the freight yard in Elgin.
British Transport Police (BTP) officers are seeking witnesses after the incident in the early hours of Wednesday.
The fire was discovered by a security guard shortly after 4am and quickly extinguished by the fire and rescue service.
Constable Mark Laurie, the BTP officer in charge of the enquiry, said: "Our information is that this was a deliberate act and I am appealing for anyone who was in the area of Maisondieu Road and Station Road in Elgin around the time the fire was reported to contact us as a matter of urgency."
British Transport Police can be contacted on 0800 40 50 40 or by texting 61016 – quoting reference B6 of 23/10/2013.
Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.