In A Tale of Two Cities, how is it that Darnay is able to pass the guard-house in France?

It is clear that if we look at the first chapter of Book
the Third, entitled "In Secret," we find that Darnay experienced many problems during
his journey to reach Paris to try and save the life of the faithful Gabelle. Clearly,
Dickens presents to us a picture of France in chaos, where the established order of
things has been overturned and has vanished, and therefore it is very difficult for
somebody such as Darnay to travel unmolested and unhindered. Note what the text tells us
in this chapter about his journey:

readability="15">

This universal watchfulness not only stopped him
on the highway twenty times in a stage, but retarded his progress twenty times in a day,
by riding after him and taking him back, riding before him and stopping him by
anticipation, riding with him and keeping him in charge. He had been days upon his
journey in France alone, when he went to bed tired out, in a little town on the high
road, still along way from
Paris.

We are told that it
was only the production of Gabelle's letter that had managed to get him this far, and he
had been forced to use the letter again to pass the guard house that had held him for so
long. The atmosphere of approaching terror and doom thus is created, which prepares us
for Darnay's arrest and his escort to Paris.