This can be done by Player and AI.
While you can't operate units to cut-off places without rail connection, new units from the purchase queue can be placed in every city with enough suply regardless of any kind of rail or road connection. This is a general game mechanic, present in SC:GC and SC:WW1.
And I aggree with numdydar, it's quite unrealistic.
In this screen you can see the deployment possibilities für China:
While the Capital is still Chungking, isolated from the rest of China, all new units can be placed in every city. Even in Yarkand and Khotan further to the west.
I can send you this save if you like.