可能

可能 alone is a shrug: 'possibly'. 会 alone is a fairly confident call about the future: 'she will'. Stack them and you get a hedged prediction, which is exactly the mood here. The order is locked: 可能 first, 会 second. Swap them and native ears hear a glitch. Drop 会 entirely and 她可能坐火车回上海 just means the option is on the table, not that it's likely. "但是她可能会坐火车回上海。" ── [《去上海》](/read/train-or-plane-to-shanghai#s8)

不 talks about something that hasn't happened yet, or a general habit. 没 talks about something already over that didn't happen. 可能 just wraps doubt around whichever one you picked. Use 不 about tomorrow and you sound like a time traveler with 没. Use 没 about tomorrow and people will assume you mean last week.

吧 is the softener that turns a flat guess into a friendly one. So the natural reply is 可能吧, the spoken version of a half-shrug. If the question used 会, echo it back: 可能会吧 sounds smoother than 可能吧 alone in that spot. In WeChat, people often add a reason right after, because a bare 可能吧 can read as cold.

可能 usually sits right before the action word, but it can also slide to the very front and cover the whole sentence. That fronted version is everywhere in speech and texting, so it won't sound weird. What breaks is parking it after the verb, which is a straight English transfer. With 会 in the mix, the order stays 可能 then 会, never the other way around.