In WitE supply can be drawn from any hex containing a working rail connected to the wider rail network (i.e. a green rail hex) and in effect supply movement within the rail network is unconstrained. From a functioning rail hex (in most cases) supply then travels by truck first to a HQ and then from the HQ to a Unit in a two phase process. In WitW this does not happen.
In WitW Logistics Supply moves around the map during the log phase as 'Freight' and it is tracked. Freight is created, stored and transferred between a network of Depots. Because freight is tracked through the system Depots close to the frontline have finite supply. Units can only get Freight from a Depot - putting a unit on a rail hex in WitW achieves nothing. Units will always try to get their Freight from the nearest Depot first. When Freight gets to a unit it is converted into one of the four types of supply - Supplies, Fuel, Ammo or Replacements - but only if they exist in the production pools. (I think of it as cashing a cheque)
So in summary whilst in WitE you had rail hexes close to the frontline with almost unlimited supply and HQs to bridge in gap in WitW you need to look to your nearest depot that may only receiving a trickle of the freight you might like and/or need.
The screenshot shows the logistic view - Depots are the triangles with the bars showing freight received (green), stored (blue), sent (red) and capacity (black). Note the different rail colours showing those used the most. Freight competes for space on the rails with moving units so you might be able to get a unit to the front and then not be able to supply it.
