The mission planners could have checked the commuter rail schedule, or arranged to attack that particular bridge on a Sunday morning.
Yes, I’m 100% sure they decided to attack that bridge right about that time. The USAF is fairly good about things like that.
Wether a train would come along at that exact moment is problematic, at best. Unless, of course Mussolini returned from the dead and caused the trains to run on schedulue during a war.