Sometimes, the trains are disrupted for engineering works, usually over Christmas and New Year when fewer people are travelling on expensive peak time commuter tickets that would be costly to lose out on or refund. When they announce the disruption in advance, the standard message is something like this:
There will be no trains on the following days. Please make a Plan B for your journey.
Here's an example of exactly this:
You may need a plan B to get to London at bank holidays and some working days. More details closer to the time, but be aware of these dates pic.twitter.com/EjveOsHWjd— Southeastern (@Se_Railway) January 31, 2017
I was thinking about this once because there was no other way for me to make my journey, so my only option was simply to not go. This doesn't count as a Plan B, though! Plan B can only be an alternative means of achieving the same ends as Plan A.
So let's say that Plan A was to catch a High Speed Southeastern train to London on May Day bank holiday. This isn't possible because they're doing some works and there are no High Speed trains on that line that day. A suitable Plan B would be to catch a slow train to London on that day. It isn't quite as good but it achieves the end of going to London that day. Or you could get a bus, if there were no trains at all, or you could drive, if you were able to. All would get you to your chosen destination on your chosen day.
Would getting the train to London on another day count as a Plan B? It might, but only if you could do the original thing you were going to do. Maybe you were going to meet a friend, and you decide to do it on Tuesday instead of Monday. I think that would just about count as a Plan B for your original intentions. But if you were going to an event on that Monday, then going another day doesn't allow you to do what you were going to do, so it's not a good Plan B.
Staying at home instead, as I had to do when I first mused about this announcement, definitely doesn't count as a Plan B for your original journey - you haven't managed to make the journey at all. But it would count as a Plan B for your holiday plans, for instance. Let's say Plan A was to go and spend Christmas with your parents, but because of work and trains, you can't get there, so you decide to spend Christmas at home. That's a Plan B for the Christmas holidays, but not a Plan B for your journey. The difference is what is under discussion, and therefore what Plan B is an alternative to.