Followed by the shade of the envious French;
Followed by fighters, in water or trench.
Before a stroke limit, unruly or weak;
Before a sour citrus, performing at peak.
Its path involves rails, deep under the ground;
Or even machines, playing extra deep sounds.
When allowed, it may rent you a flat or a house;
But when mixed, it's the port for a cable or mouse.
Before a book's name, helps the foreign and deaf;
Alone, it's a shorthand for a deli-bound chef.
What am I?