The Late Roman Army

[Aryaman2]

Of course, the premise is false, as has been said, cavalry is not faster than infantry at strategical level, any cavalry force had a baggage train that moved at wagon speed.

Unless your 'supply' train of fodder and fresh horses are already ahead of you in strategically placed forts/locations.

I think this must be what's happening.

This is in essence, the feudal model of logistics, heavily armed knights and well fortified strategically placed supply stations. I think the Medieval feudal military model is a direct outgrowth of the Late Roman system.

The first thing Charlemagne does is rebuild the forts (many of which are monasteries) What good are the forts if the army is 'mobile' anyway? They are the supply chain.

The system of a chain of supply depots in the route of an army was, as far as I know, the exception, not the rule, in medieval times. I know the English kings did that in the Scottish campaigns, and to some extent in the 100 years war, but I can think of very few other examples. The rule is that medieval knights ideally were expected to carry at least 3 horses with them.
The system you are talking about was instead fully developed in Western Europe in the XVI and early XVII century, the problem with that system is that it only provides supply in friendly areas. Besides I have never heard of that a system used either by the Parthians or by the Sassanians, do you have any source on that?