People who were forced from their homes after a train derailed in central Alberta are now back and Canadian Pacific Railway says it's investigating what caused the wreck.

A highway and rail line south of Lacombe, Alta., had to be closed for several hours to allow for cleanup after litres of tar oil spilled from a derailed rail car.

Lacombe city officials say 22 rail cars were derailed along Highway 2A on Friday night, mainly carrying asphalt oil.

They say one rail car released 32,000 litres of tar oil.

The city says no nearby residents or Canadian Pacific Railway employees on the train were injured.

It says residents who were evacuated are now back home.