Standard loaves of bread tend to be too fragile for travel. They squish, become soggy, and seep the sandwich contents everywhere. A baguette is the far superior choice for travel, because it's easy to hold and has a hardy crust that can soak up moisture.
Here's the secret to finding the perfect baguette: choose one that is individual-sized with a soft, spongy interior and a sturdy crust. Avoid sourdough on the drier side; you want the baguette to be crusty but malleable enough to bite without using your other hand to help tear off the bread. Remember, this is a one-handed sandwich for the road!