Re: How can knowing which platform a train leaves from be a security problem?
I can think of several ways I could abuse this knowledge if I were so inclined, generally involving timers planted well ahead of time.
If I were inclined to target a specific train at a certain time I'd just choose another station elsewhere on the line where there isn't the opportunity to switch platforms at the last minute. Or anywhere the multiple tracks into platforms converge into fewer.
Or plant my device at departure, or along the journey, or at whichever platform it does eventually arrive at.
It seems to me that any reduction in genuine security threat by not announcing in advance which platform will be used is at best minimal and more a convenient excuse for not knowing.