Okay finally an on topic logistics post from I the collector of acorns, nuts, berries, birds' eggs, nubile females etc
Under the Marian reforms there seems to have been a deliberate move to reduce the army's reliance on wheeled transport for the carriage of the basic tools and equipment issued to the troops. Instead each Roman legionary (and likely most auxiliary foot soldiers) was issued a forked stick from which he would hang much of his gear on the march. In addition each contubernium of eight men were issued a mule to carry the rest.
In addition at least some soldiers kept servants, (you must not automatically assume all of these were slaves) but I am not sure from my materials if these were limited to the principals (Roman officer and NCO equivalent grades) or if common soldiers were also allowed to follow this practice.
Further the army itself owned a stock of slaves called galearii (helmet wearers) who seem to have been issued some uniform and seem to have managed the baggage train.
Note that while the aim was to get away from wheeled transport this was not entirely possible. Each century was issued a carroballista (oddly enough this means a ballista on a carriage) and each cohort an onager which was a form of torsion powered catapult. These and their other gear would have been carried on wheeled carts.
In addition while as Naraku states forage was always an option it does tend to slow the rate at which an army can march. Thus often the Romans seem to have relied on organising markets ahead of their arriving legions in territories with sympathetic populations or something similar to the magazine system employed by the French in the gunpowder era.
When relying on a central magazine or similar supply point Roman armies tended to keep close to water as this was by far the most effective means of transporting cargo, so campaign would adhere to rivers and or coastlines. Wheeled transport was far more limited being animal drawn by horses, mules, donkeys or oxen and needed good going preferably roads. There is a reason that the wilds of places like Wales and Scotland and the German interior proved difficult to conquer.
I have actually found a rather lacklustre time finding hard sources on how the Romans actually assembled and transported military captives. The Roman writers are by and large great on rhetoric, crap on detail.
This picture of a marble relief in modern Turkey and similar ones found in Mainz do support the use of coffles at least for small parties of captured slaves.
We also have the works of Julius Caesar and others indicating that professional slave traders followed campaigning armies. What we lack as always are actual details.
Well he was a very important and influential figure in the early church,
