It's not like a mall
> There is no multiplier effect
Your multiplier effect is the initial factor advantage of how easy it is to pack a firearm on a train.
A depressed kid could do it, easy target to recruit online, and doesn't even need firearm training considering that it would be as easy as shooting sardines in a barrel given the physical space in a train.
So basically an ill intentioned group would need to recruit 20 depressed kids , put them on 20 different trains departing at approximately the same time and they'd cause a toll in excess of everything seen before
A train is not a mall.
What if the train stops? This can be done by any personell quickly. Doors and windows of trains can be opened everywhere, they are designed that way. Stop the train, intercom and tell people to leave the train and the situation is over.
Chances are this will turn out differently than you assume
So I stand by what I said and I claim once again that you only need one ill intentioned person with a firearm and everybody on board is basically done, they'd move across compartments and off everybody.
There are thousands of trains which all have that same vulnerability, so a coordinated attack would have a toll much higher than everything seen before.
People have a blindspot, they'd claim that such attack would never take place because the "production value" is not the same as explosive attacks.
I don't think hoping that ill intentioned people hold "production value" as rigorous parameter when planning an attack is a valuable strategy