To cross the bridge, the train must travel a total distance of (train length + bridge length) = 1 + 1 = 2 miles. It does this in 1 minute (60 seconds). So its speed is 2 miles/minute. To pass a signal post, it must travel its own length, which is 1 mile. Time = Distance / Speed = 1 mile / (2 miles/minute) = 0.5 minutes = 30 seconds.