The rear projected background railroad scenes were filmed in black and white, and then tinted. They were filmed around Burbank on the SP in 1946. Information from Wes Clark's page: http://wesclark.com/burbank/1946_trains.html
There are links on Wes' site to the raw footage posted to YouTube. There is discussion on YouTube on why the background scenes were filmed in B&W. I think that the slow speed of the color movie film of the period would not have rendered well at night. There is not supposed to be much color in a night scene anyway.