So... I've swatched (to get an idea of how to do the brioching thing), I've cast on, and I've made sure the travel project actually does fit the "travel project requirement" - which is, coincidentally, "fitting into the project bag".
You can see all the stuff that lives in there, permanently or temporarily. Permanent inhabitants are: measuring tape, a pencil (with eraser endcap), a needle gauge/ruler, endcaps for the interchangeable needle cables, a bit of string, my row-counting ring, the little bag for stitch holders and darning needles and cable needles, and last but of course not least an emergency cookie and emergency chocolate. (You never know.)
Non-permanent residents are the needles for the current project, the yarns for it and the printed instructions (or in some cases, when the instructions are on my phone only, a bit of paper to scribble notes on).
Unfortunately, the bag is not bigger on the inside, probably because it's the Exploding Tardis motif and not the proper, normal, intact one.
In any case, to be deemed a travel project, the thing has to fit inside. In this case, it just does:
So. Train pastime - checked.
And since this is rather slow, meditative knitting, I will surely not run out!