[9] It had departed from Chicago at 10 a.m. the prior Friday (Dec 29), bound for San Francisco[3] and normally traveled in one long section, but on this occasion it was split into two, with the passenger train running ahead of the mail express.
Early that morning an unusually long and heavy freight train developed problems (unofficially a hot box)[3][10] whilst traveling west from Ogden.
[3] First 21 departed Ogden at 4:38 a.m. (38 minutes after its scheduled departure) and had slowed to 8 mph (13 km/h) at the time of the collision, preparing to stop in response to the flagman's signals from the preceding freight train.
Cars of the mail express section piled up crossways of the track behind the engine, some of them sliding down the causeway embankment into water.
[5]: 7 At the site of the crash, the tracks run along a causeway across desolate mud and shallow water, so all rescue efforts had to come by rail.