Rail redeployment can bring distant cavalry through friendly territory to the front more quickly. It costs the units organization in the game, reducing their immediate readiness to fight. That penalty is accepted here because the plan calls for movement and occupation instead of combat. Units several provinces behind the advance are therefore told to use the railways first.
Notable Point · 2:12:49-2:13:06
Rail speed matters more than combat readiness for this advance
Cavalry can accept the organization loss from rail redeployment because its immediate task is to move and occupy territory.