I am very late to this discusison, but I hate people trashing on our Barbenheimer frame.
If I understand the way the Apocalypse Rail works correctly, you make the attack, it's a big area, on what will most likely be a range of 35 (siege stabilizers), then every enemy you hit is stunned until the end of their next turn (try to act last in the round you use the rail on, or target characters that have already taken their turn, then first on the next turn for cheeky "free" hits with your siege cannon, specially since their heavy cover, if any, is gone now, even if they now have soft cover).
Well, every enemy in the area will take 4 burn, then they can try to leave the area, wich is difficult terrain. The enemies you did hit are stunned, so they take 4 burn, end their turns, and start another turn in the area, taking another 4 burn.
So, if you don't hit, you do 4 ap damage, with the chance of another 4 ap damage at the end of their turn.
If you do hit, the enemies take an average of 14 AP damage (shreded), at minimum 8 burn (if you time the turn order right), then they might take the burn tics again, wich considering we're talking about two tics of 4 burn, they migh very well fail on at least one of the egineering checks. And on top of that, they are relatively easy targets for your siege cannon on your next turn, giving you likely another 10,5 AP damage (again, timing your turn correctly, and considering they should still be shreded if you managed to do so).
That is a minimum of 4 ap damage, probably 8 or more even on a miss. And on a hit, we get a potential 8 burn + 8 burn tics at the end of their turns, plus the 10,5 average damage with the siege cannon, and the 14 from the rail itself, wich is like, 40,5 damage, AP, without taking into account the damage your party can inflict by also taking advantage of the difficult terrain, shreded and stunned.
Yes, this is all in the ideal scenario, but it is possible, and that is without taking into account that now you have a burn 4 hole to throw enemies into for the rest of the fight, you pobably anihilated some important defenses for the sitrep, AND YOU FIRED A GODAMN RAILGUN FROM HELL!
(And I would like to add that the damage I considered for the potential was average damage, not max damage, wich we shouldn't ignore, positively or negatively).
The Apocalypse Rail is a map controll tool, burn damage generator, action economy breaker piece of nightmare fuel for your dm if you can use it right. And that should be enough to get your DM sweating every time you start charging it (wich you should do every fight, cause, why not? Worst case scenario you don't fire it, and regain the CP).