Space! Train battery boxes are huge:
The alternative is 9v, but it's very expensive (average €38 for a used 9v train motor on Bricklink, but then you have to buy 9v track as well - the Circuit Cubes kit is £35 and no 9v track required).
The mid section of my diesel will be only 4-wide, so it's 2-wide on the inside, so no room for a battery box.
I've redesigned the diesel to fit in circuit cube motors (2 if necessary, as the kit I'm looking at comes with 2 anyway, but I'll try it with one). You can see motors marked M and battery marked B here: