Random Thought - Trains of Thought

So there you are, plugging away at your latest novel, the world you've imagined being vividly detailed on the computer screen before you and then it happens, the phone rings or someone calls your name or there's a knock at the door.

Your train of thought has just derailed and crashed into a heap of twisted metal that you now have to go back to and try to salvage what you can of your world.

It has happened countless times to me, in fact I should have smoke spewing out of my ears for as many trains of thought that have crashed inside my head. Usually what happens to me is the TV that I can usually tune out, says something so off the wall it breaks into my brain. The perils of having my computer next to the family TV.

So I'm curious if this situation happens to anyone else and if so, what do you do to get back on track?